ThreadPoolBuilder: do not use a shared work queue.
create a subclass that wraps a guage that tracks worker queue size.
Break InstrumentedThreadPoolBuilder out into its own class
expose keepalive time in ThreadPoolBuilder.
Use instrumented threadpools throughout.
Merge pull request #144 from gdusbabek/tp_builder_reusable

Make ThreadPoolBuilder reusable.
Merge branch 'master' into expose_preaggregated_cfs