HAMMER 32/many: Record holes, initial undo API, initial reblocking code

HAMMER 32/many: Record holes, initial undo API, initial reblocking code

* Add code to record recent 'holes' created by the blockmap allocator
  due to the requirement that data blocks not cross a 16K hammer buffer
  boundary, in order to try to fill in the gaps with smaller chunks of data
  when possible.

  Currently a hole is not added for blockmap frees.  It is questionable
  whether it is a good idea to do it for frees or not, because it can
  interfere with the reblock code's attempt to completely free a big block.

* Add a reblocking ioctl which scans the B-Tree and reblocks leaf nodes,
  records, and data in partially empty big blocks to try to free up
  the entire big block.  Incomplete (needs to reblock internal B-Tree nodes
  and doesn't yet, needs a low-free-space mode which focuses on freeing
  a single large block).

* Add the API infrastructure required to implement the undo records,
  and implement the initial undo code (sans ordering requirements for
  writes).  Incomplete.