Add strong rwlock consistency checks to dns_rbtdb

Add strong rwlock consistency checks to dns_rbtdb

The dns_rbtdb unit already tracks the state of the node and tree rwlocks
during the top level function and passes the states of the locks to the
called functions.

Add the tree locking family of macros modeled after node locking macros,
and expand both to track the state of the lock in an external variable.
Additionally, in developer mode, add precondition to the macros, so the
lock is in required state - this should cause an assertion failure on
double locking instead of the thread getting stuck.