implement multiplexer for scalac

[wix-andriusb]

Description

Add multiplexer support for the scalac worker.

This creates a thread pool inside of the scalac worker and uses threads in order to use a singular process.

Motivation

Potential speed improvement and memory usage improvement because we use a single JVM process which warms up faster.

Implementation

The current implementation passes dedicated output streams to each worker which collect the output and then returns it to bazel.

[wix-andriusb]

@simuons I created a dedicated MultiplexWorker.Interface and made only ScalacWorker utilize it, this means other workers can migrate independently to this new interface.

[JackTreble]

Thanks @wix-andriusb for working on this I was starting to investigate how to do this change, @johnynek @simuons what is changes are outstanding before this can be approved? :smile:

[wix-andriusb]

It did not make the compilation faster.

My guess is that it is bottlenecked by re-packing the jars.

[liucijus]

closing as stale, feel free to reopen