HAMMER VFS - REDO implementation base code part 2/many

HAMMER VFS - REDO implementation base code part 2/many

* Move hammer_generate_redo() to its own source file, hammer_redo.c

* Fix bug in the REDO generation.  The tail type was not set the same
  as the head type and caused recoveries to fail.

* Flesh out the REDO sequencing by adding REDO_TERM_* records during the
  meta-data flush, allowing REDO_WRITEs and REDO_TRUNCs to be matched
  against REDO_TERM_WRITEs and REDO_TERM_TRUNCs.

* Interlock the writing of the root volume in the flusher.  Frontend
  code modifies the root volume when generating REDO records and
  can collide with the flusher.