thread name for the workunit streamer

[cburroughs]

This is so it shows up more clearly in stacktraces or perf.

Test Plan

In it this repo:

PANTS_WORKUNIT_LOGGER_ENABLED=true pants check ::

While running:

gstack $(cat .pants.d/pids/35b2225442f6/pantsd/pid ) > /tmp/stack
$ grep worku /tmp/stack
Thread 2 (Thread 0x7ee0728296c0 (LWP 426293) "workunit-stream"):

[sureshjoshi]

This should have a release note, as I think people in chat have mentioned using this - and so if anyone else put automation around this, they should know they might need to change it