Added the isc_refcount_t type, which is a generic locked reference counter,

Added the isc_refcount_t type, which is a generic locked reference counter,
and make the rbtdb, zone, and view objects use it.
This should reduce contention on other locks, since (in the normal)
implementation, the reference count has its own lock.  In the future, it
should also be possible to implement an isc_refcount_t with atomic operations
instead of mutexes, which should also help performance.