[sgen] Sleep a little while busy-waiting for the gray queue distribution.

[sgen] Sleep a little while busy-waiting for the gray queue distribution.

In the parallel collector, the main GC thread needs to distribute all
remaining entries in the gray queue to the worker threads.  Since the
distribution pool is limited in size we need to wait on it, which at
this point is only possible with busy-waiting.  Since we don't want to
use up CPU resources and hog the cache lines we wait 2ms between
distribution attempts.