Investigate & resolve nondeterministic build errors

[sming]

• I am a heroic developer
• Who wants to have deterministic builds
• So that I can retain my sanity and be productive instead of chasing random build errors

Design & Implementation Notes

• See Heroic Build Issues Analysis gdoc
• See this Slack thread for details
• See this build for a full technical example
• be aware of network errors that CircleCI seem to have
  regularly (but not always)
• and other weird, random issues that CircleCI seems to throw up
• I think this is occurring irrespective of the branch too (but might only pertain to feature/add-bigtable-timeout-settings-refactored) :

feature/add-bigtable-timeout-settings-refactored com.spotify.heroic.GrpcClusterQueryIT > distributedFilterQueryTest FAILED
    java.lang.IllegalStateException: failed to create a child event loop
        Caused by:
        io.netty.channel.ChannelException: failed to open a new selector
            Caused by:
            java.io.IOException: Too many open files
    java.lang.NullPointerException

[sming]

Update

found that removing the 4x multiplier from this method :

        @Provides
        @GrpcRpcScope
        @Named("worker")
        fun worker() = NioEventLoopGroup(Runtime.getRuntime().availableProcessors() * 4)

stops the exception from being thrown. But there are still many questions unanswered:

1. why only my and Sergey's machines
2. why does commenting out a seemingly innocuous IT (testbasicWithNoDistribution) also stops exception from being thrown
3. is the problem we're encountering pertinent to production operation of Heroic or is it a quirk of our Machines or just something that unit test code will exhibit
4. what is the "correct" fix for this. Removing the 4x multiplier is a poor workaround at best.

(CC @malish8632)