wip: Created stub of Executor mixin module.
wip: RubySingleThreadExecutor now gets run/stop/kill from Executor mixin.
wip: Java executors now share JavaExecutor mixin module.
Reset ImmediateExecutor to original state.
Reset PerThreadExecutor to original state.
wip: RubyThreadPoolExecutor now uses Exector for stop events.
wip: RubyThreadPoolExecutor now uses post/execute from Exector.
wip: Replaced instanced of 'thread pool' with 'executor' in Executor YARD doc.
More consistent shutdown/kill behavior for executors.
TimerSet now posts through Executor.
Shared #shutdown for RubySingleThreadExecutor, RubyThreadPoolExecutor, and TimerSet.
Most executors now share behavior through the Executor and JavaExecutor mixin modules.