Build fail with redis feature

Hi. I'm getting errors while trying to build `flowgger` with `--all-features` (rust 1.62.0) as pointed out in https://gitlab.alpinelinux.org/alpine/aports/-/issues/13952#note_243393.

Changing the declaration to `let mut ...` (like the log suggests) makes the build pass but I honestly don't know whether things will break. This seems to be happen after `redis-rs` 0.11.0-beta1 release (ref: https://github.com/redis-rs/redis-rs/blob/main/CHANGELOG.md#0110-beta1---2019-05-30)

```rust
warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tcp/tcpco_input.rs:52:18
|
52 | decoder: Box,
| ^^^^^^^
|
= note: `#[warn(bare_trait_objects)]` on by default
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
52 - decoder: Box,
52 + decoder: Box,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tcp/tcpco_input.rs:53:18
|
53 | encoder: Box,
| ^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
53 - encoder: Box,
53 + encoder: Box,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tcp/tcpco_input.rs:29:22
|
29 | decoder: Box,
| ^^^^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
29 - decoder: Box,
29 + decoder: Box,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tcp/tcpco_input.rs:30:22
|
30 | encoder: Box,
| ^^^^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
30 - encoder: Box,
30 + encoder: Box,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tls/tlsco_input.rs:52:18
|
52 | decoder: Box,
| ^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
52 - decoder: Box,
52 + decoder: Box,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tls/tlsco_input.rs:53:18
|
53 | encoder: Box,
| ^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
53 - encoder: Box,
53 + encoder: Box,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tls/tlsco_input.rs:29:22
|
29 | decoder: Box,
| ^^^^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
29 - decoder: Box,
29 + decoder: Box,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tls/tlsco_input.rs:30:22
|
30 | encoder: Box,
| ^^^^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
30 - encoder: Box,
30 + encoder: Box,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tcp/tcpco_input.rs:61:51
|
61 | "capnp" => Box::new(CapnpSplitter) as Box>,
| ^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
61 - "capnp" => Box::new(CapnpSplitter) as Box>,
61 + "capnp" => Box::new(CapnpSplitter) as Box>,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tcp/tcpco_input.rs:62:49
|
62 | "line" => Box::new(LineSplitter) as Box>,
| ^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
62 - "line" => Box::new(LineSplitter) as Box>,
62 + "line" => Box::new(LineSplitter) as Box>,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tcp/tcpco_input.rs:63:53
|
63 | "syslen" => Box::new(SyslenSplitter) as Box>,
| ^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
63 - "syslen" => Box::new(SyslenSplitter) as Box>,
63 + "syslen" => Box::new(SyslenSplitter) as Box>,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tcp/tcpco_input.rs:64:47
|
64 | "nul" => Box::new(NulSplitter) as Box>,
| ^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
64 - "nul" => Box::new(NulSplitter) as Box>,
64 + "nul" => Box::new(NulSplitter) as Box>,
|

warning: use of deprecated associated function `may::Config::set_io_workers`: use `set_workers` only
--> src/flowgger/input/tls/tlsco_input.rs:33:23
|
33 | may::config().set_io_workers(tls_config.threads);
| ^^^^^^^^^^^^^^
|
= note: `#[warn(deprecated)]` on by default

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tls/tlsco_input.rs:68:51
|
68 | "capnp" => Box::new(CapnpSplitter) as Box>,
| ^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
68 - "capnp" => Box::new(CapnpSplitter) as Box>,
68 + "capnp" => Box::new(CapnpSplitter) as Box>,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tls/tlsco_input.rs:69:49
|
69 | "line" => Box::new(LineSplitter) as Box>,
| ^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
69 - "line" => Box::new(LineSplitter) as Box>,
69 + "line" => Box::new(LineSplitter) as Box>,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tls/tlsco_input.rs:70:53
|
70 | "syslen" => Box::new(SyslenSplitter) as Box>,
| ^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
70 - "syslen" => Box::new(SyslenSplitter) as Box>,
70 + "syslen" => Box::new(SyslenSplitter) as Box>,
|

warning: trait objects without an explicit `dyn` are deprecated
--> src/flowgger/input/tls/tlsco_input.rs:71:47
|
71 | "nul" => Box::new(NulSplitter) as Box>,
| ^^^^^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see
help: use `dyn`
|
71 - "nul" => Box::new(NulSplitter) as Box>,
71 + "nul" => Box::new(NulSplitter) as Box>,
|

error[E0596]: cannot borrow `redis_cnx` as mutable, as it is not declared as mutable
--> src/flowgger/input/redis_input.rs:105:46
|
99 | let redis_cnx = self.redis_cnx;
| --------- help: consider changing this to be mutable: `mut redis_cnx`
...
105 | let dummy: RedisResult = redis_cnx.rpoplpush(queue_key_tmp, queue_key);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable

error[E0596]: cannot borrow `redis_cnx` as mutable, as it is not declared as mutable
--> src/flowgger/input/redis_input.rs:110:38
|
99 | let redis_cnx = self.redis_cnx;
| --------- help: consider changing this to be mutable: `mut redis_cnx`
...
110 | let line: String = match redis_cnx.brpoplpush(queue_key, queue_key_tmp, 0) {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable

error[E0596]: cannot borrow `redis_cnx` as mutable, as it is not declared as mutable
--> src/flowgger/input/redis_input.rs:117:40
|
99 | let redis_cnx = self.redis_cnx;
| --------- help: consider changing this to be mutable: `mut redis_cnx`
...
117 | let res: RedisResult = redis_cnx.lrem(queue_key_tmp as &str, 1, line as String);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable

For more information about this error, try `rustc --explain E0596`.
warning: `flowgger` (lib) generated 17 warnings
error: could not compile `flowgger` due to 3 previous errors; 17 warnings emitted
```

Start with src/flowgger/input/redis_input.rs around the redis_cnx declaration at line 99 and the failing Redis calls at lines 105, 110, and 117. Run the reported Rust 1.62.0 build with --all-features, then verify that the Redis feature builds successfully without changing its runtime behavior.

Written by the indexing model from the issue text.