WIP Threading: Attempt to hide/replace UserThread nested class

[msgilligan]

This is partly an attempt to remove use of Guava's Uninterruptibles.putUninterruptibly and Uninterruptibles.takeUninterruptibly, but also to see if simplification is possibly by using a built-in JDK Executor rather than using a custom one.

The first and second commit could possibly be cherry-picked and merged.