Don't release the tree read lock in dereference_iter_node()

Don't release the tree read lock in dereference_iter_node()

Previously, the tree read lock could be upgraded to a write lock in
decrement_reference() and then downgraded back to read lock in
dereference_iter_node().  When the use of isc_rwlock_downgrade() was
removed, the downgrade was changed to a simple unlock+lock. This allows
some delete operations to sneak in and delete nodes that the iterator
expects to be in place.

Expand decrement_reference() so the caller can indicate whether the
tree read lock should be upgraded, and disallow the upgrade when
calling from dereference_iter_node(), so there will be no need to
release the lock afterward.