hammer2 - freemap part 2 - cleanup & performance pass

hammer2 - freemap part 2 - cleanup & performance pass

* Increase minimum I/O from 1K to 16K (HAMMER2_LBUFSIZE).

* Adjust some use cases, increasing the smallest indirect block to the
  minimum IO size 4K->16K.

* The BREF_TYPE_FREEMAP_LEAF bitmap is now embedded in the chain, allowing
  us to use the 256 byte bitmaps more safely.  As with inodes, a read-in
  involves using a larger buffer-cache buffer and then copying the data
  into the chain.

* Avoid unnecessary reads from disk when allocating blocks that are less
  than the minimum IO size.  If it is determined that the bitmap bits
  covering the minimum IO size are 0 (unallocated), we pre-validate the
  larger buffer cache buffer so the later bread() does not have to issue
  any actual reads.

  Pre-validation for allocations larger than or equal to the minimum IO
  size is already handled in the chain locking code.

  With this change, a cpdup into H2 will issue mostly writes and is able
  to avoid nearly all reads.

* Make some progress formalizing chain-embedded data.