HAMMER 12/many - buffer cache sync, buffer cache interactions, misc fixes.

HAMMER 12/many - buffer cache sync, buffer cache interactions, misc fixes.

As of this commit all vnops work, the filesystem remains consistent across
umount/remount, and the 'cpdup / /mnt/root' test succeeds.  There is still
a ton of work to do but this is a major milestone.

* Get sync and umount working properly.  Properly sync out in-memory records
  and inode updates, and properly release buffer cache buffers so the
  kernel can throw them away.

* Modifications to buffers backed by the buffer cache ensure that any
  asynchronous writes complete to avoid a modify-write race.

* Fix miscellanious reference counting bugs.

* Fix two insertion bugs for internal B-Tree nodes that were causing the
  code to blow up.