hammer2 - Initial synchronization thread

hammer2 - Initial synchronization thread

* Add infrastructure for a per-PFS synchronization thread.  This primary
  thread will be responsible for background synchronization out-of-sync
  masters, slaves, and other elements of a PFS.  There will also be another
  thread for background bulkfree and dedup operations.

  In this commit, basic thread creation, destruction, freeze, and unfreeze.
  (Freezing and remastering is required when new nodes are added to a PFS
  or existing nodes are deleted).

* Define new cluster flags indicating the synchronization state.

* pfs-create and snapshot were not properly flagging the inode as being
  a PFS, which confused the on-mount recovery scan code.

* Update TODO.