LockSet: forbid add() on a partially owned set

This patch bans add() on a half-acquired set. This behavior was
previously possible, but created a deadlock if someone tried to acquire
the set-lock in the meantime, and thus is now forbidden. The
testAddRemove unit test is fixed for this new behavior, and includes a
few more lines of testing and a new testConcurrentSetLockAdd function
tests its behavior in the concurrent case.

Reviewed-by: imsnah