Remove lock_and_signal::lock_held_by_current_thread

[brson]

This is used for two purposes: 1) to assert that we aren't retaking a lock that we already own and 2) to enable a single recursive lock in rust_task_thread.

This functionality entails calling pthread_self a lot which is not free and imposes significant overhead (maybe 30%?) on all our locking.

To get rid of this we would need to get rid of the one recursive lock. Probably we would want to leave it in debug builds because it does some very useful sanity checks.

[brson]

The recursive lock is gone now.

[brson]

Fixed by b247de6