Previously this just turned the rw_lock into the equivalent of a
recursive_lock, which meant that reader vs. writer assertions
were of no use.
Now, we have a per-thread static array which stores the held read
locks, allowing ASSERT_READ_LOCKED_RW_LOCK to work properly,
and allowing multiple readers to be active at a time.
This probably should still remain disabled even on nightly builds,
but at least it's much more useful as a debugging tool than it was
beforehand.
Change-Id: I386b2bc2ada8df42f4ab11a05563ef22af58e77f
30fda09a95