fix yielding regression between readers and writers

After version 2.2, the sleep for yielding reader threads was removed. This caused
reader threads to be able to regain their shared lock before a writer could
gain an exclusive lock, even when there was a queue of pending writers.
By adding back this sleep of a tiny amount, this has the effect of ending the
reader thread's quantum, and thus makes our lock implementation appropriately
writer-greedy.

This should fix query_yield1.js and query_yield2.js, as well as other assorted
unit test failures.