wasmtime-cache: file-per-thread-logger should be an optional dependency

#### Feature

Make `file-per-thread-logger` an optional dependency for `wasmtime-cache`.

#### Benefit

`wasmtime-cache` pulls in `file-per-thread-logger`, which also brings `termcolor`, `regex`, and `atty`, even if you don't want to have those logs.

This is especially relevant because `cache` is a default feature of `wasmtime`.

In addition: glancing at the code it seems like the logging feature is actually never used - `Worker::start_new` is always called with `None` as the log prefix.

Is this some left-over code?