hammer2 - Stabilization pass
* Fix case in hammer2_chain_delete_duplicate(), allow delete-duplication
of a deleted terminal chain outside of a flush.
* Fix assertions in the flush code. Deleted parent's block tables are not
updated. If all parents are deleted we allow the CHAIN_DELETED flag to
be cleared in children even though the children might still be on the
DBTREE or be flagged BMAPPED.
* Fix bug in hammer2_bioq_sync() where new logical buffers instantiated while
flushing existing buffers are not waited for before exiting the PREFLUSH
state. This might not actually be a bug but cover the case for now.