Inconvenient logging levels and logging target formats

In a context of a larger application AlephBFT crate logs a lot of errors on high levels for things that are normal and expected during typical use, even if "an error", or "warning" in the context of AlephBFT's internals. Please note that rust logging levels are global and shared for all the crates of the application, so the deeper "in the stack" given library is, the less the chance that certain logging statement is really relevant to the end user.

These logging messages are also cryptic and not useful for anyone that is not intimately familiar with its internals.

On top of it, AlephBFT currently makes it really inconvenient to disable or silence its log messages. The `target:` in logging statements is meant to be (and defaults to) a Rust module path. So the separator use for the hierarchical structure is `::`, like Rust modules. Had it been:

```
info!(target: "alephbft::extender", ...);
```

etc. it would be natural to disable all alephbft logging with:

```
env RUST_LOG=info,alephbft=off
```

but since `AlephBFT-xyz` is used, this is not the case.