hammer2 - Start work on quorum validation.

hammer2 - Start work on quorum validation.

* Remove HAMMER2_PFSTYPE_SNAPSHOT, it adds unnecessary complication.
  Just make snapshots HAMMER2_PFSTYPE_MASTER.

  Add a subtype field to the inode and PFS ioctl to identify snapshots.

* Start fleshing out the quorum code.  hammer2_chain_lock() is the core.
  It will eventually also have to be async.

  hammer2_chain_lock() now does hard quorum checks, soft master checks,
  and identifies slaves and whether synchronization is needed, based on
  mirror_tid.

* More cleanup of cluster->focus.

* When removing nodes from a PFS cluster, since the cluster is being
  ripped apart and hammer2_inode_lock_ex() does quorum validation,
  we cannot use this function.  Just acquire and release the low-level
  inode mutex instead.