HAMMER 13/many - Stabilization commit

HAMMER 13/many - Stabilization commit

* Clean up the in-memory record API.

* Add B-Tree boundary assertions and B-Tree debugging code.

* Delay freeing bits in the allocation bitmaps for B-Tree nodes and
  clusters until the last reference to the in-memory structure goes away.
  This avoids premature reallocation.

* Fix a bug in btree_split_leaf() - the cursor was not being properly
  adjusted in a particular boundary case.

* Fix a recursive lock bug on a buffer cache buffer in hammer_io.c

* Do not allow a non-empty directory to be removed.

* Synthesize directory entries for "." and "..".  Adjust the
  hammer_directory_namekey() procedure to reserve key-space for the
  synthesized entries.

* Fix memory leaks related to the hammer_node and hammer_record structures.

* Finish writing the rename code.