light-weight read-write lock, that doesn't use a semaphore itself. Locking
and unlocking has to be done via methods of RWLockManager. This combo allows
lots of locks without risking to hit the semaphore limit.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@29686 a95241bf-73f2-0310-859d-f6bbb57e9c96
