Remove LWKT reader-writer locks (kern/lwkt_rwlock.c). Remove lwkt_wait

Remove LWKT reader-writer locks (kern/lwkt_rwlock.c).  Remove lwkt_wait
queues (only RW locks used them).  Convert remaining uses of RW locks to
LOCKMGR locks.

In recent months lockmgr locks have been simplified to the point where we
no longer need a lighter-weight fully blocking lock.  The removal also
simplifies lwkt_schedule() in that it no longer needs a special case to
deal with wait lists.