hammer2 - Rework the flusher

hammer2 - Rework the flusher

* Rework the flusher to remove numerous degenerate cases which caused
  excessive re-flushing of the same block(s).

* No longer flushes the volume header on sync if the filesystem is not dirty.

* General code cleanup.

* Beginning of a work-up to implement variable logical block sizes for file
  data.  Physical blocks will always be 64K but logical blocks and block
  allocations can be any multiple of 1K.

  Currently any file > 512 bytes allocates a 64K file block, which is
  horribly space inefficient.  This will be fixed soon.