HAMMER 14/many - historical access cleanup, itimes, bug fixes.

HAMMER 14/many - historical access cleanup, itimes, bug fixes.

With this commit most historical accesses work and I can go through at least
two cycles of cpdup / rm -rf without crashing.

* Fix a bug in the B-Tree code related to searches on historical records.
  These records are differentiated by key.create_tid but unlike the rest
  of the key the matching algorithm has to be somewhat more sophisticated.
  e.g. A search as of time 10 needs to find a record with a create_tid of 5.

  To make this work properly we use a trick when we generate the separator
  when splitting a leaf. see hammer_btree.c / hammer_make_separator().

* Recycle inodes a link count of 0 immediately.

* Optimization: Do not flush backing store to disk on reclaim.

* Add a per-inode read-only flag.  Mark all historical inodes as read-only

* Implement read-only semantics in the vnops and assert attempts to modify
  inodes marked read-only.

* Properly record the last transaction id for use when synchronizing
  inodes to the platter.  There were a few cases when late synchronizations
  were using the wrong transaction id, breaking user expectations when
  accessing historical data after a sync.

* Update the itimes (atime and mtime). These are non-historical updates by
  default (meaning we just overwrite the latest inode record instead of
  deleting it and inserting a new one).  A future mount option will allow
  these updates to operate historically (the coding is trivial)... didn't
  you always want to know the last time a file was accessed prior to a
  particular date?  Think about it...

* Fix an inode memory leak.  The inode in-memory structure must be freed
  on last reference.  There were cases where it was being left in the
  HAMMER cache.

* Optimization: Reduce vnode scan overhead during 'sync' by improving
  on the inode flags which indicate that some sort of sync is required.

* Optimization: Don't flush inodes when their link count drops by 1, try
  to only flush them when their link drops to 0.

* Fix a couple of potential deadlocks.

* Fix a case in the vnops code where an inode was not being properly flagged
  as being dirty.