hammer2 - Implelment variable-sized physical storage allocations

hammer2 - Implelment variable-sized physical storage allocations

* The physical block which straddles the file EOF now only allocates
  the nearest power of two >= 1024 bytes sufficient to hold the file.

  Note that files <= 512 bytes still fit in the inode embedded data area.

  This results in much, much better packing of small files.

* Remove most restrictions on logical/physical size mismatches.

  This also opens the door in the future for variable-sized indirect blocks.

* The strategy read code is now a lot smarter on optimizing to a direct-read.
  (direct writes are still not supported yet).  The code should also be
  more readable now.

* Rewrite the file truncation/extension code to use the new
  hammer2_chain_resize() function.

* Correct several bugs in the buffer offset calculations when pulling 16K
  logical buffers out of 64K physical buffers.