LogManager implementation is modified to use ManagedLogger, which can swap out backing Appenders without re-creating the log instance.
The State was also changed to track `currentCommand: Option[Exec]`. `Exec` knows the origin of the command invocation, and using that we can now send the network-originated events only to the network clients.
Combined together, this implements log splitting between the sbt clients (channels).
This is the beginning of a lightweight client, which talks to the
server over Contraband-generated JSON API. Given that the server is
started on port 5173:
```
$ cd /tmp/bogus
$ sbt client localhost:5173
> compile
StatusEvent(Processing, Vector(compile, server))
StatusEvent(Ready, Vector())
StatusEvent(Processing, Vector(, server))
StatusEvent(Ready, Vector())
```
Fixes#2734, Ref #1041e93c4450a1 added a feature called early
command, which uses `--` as a prefix to denote some commands that runs
ahead of session loading. While the feature might be useful especially
for logging, `--` is too useful just for this purpose.