HAMMER Util - Add new features, fix history retention bug in prune

HAMMER Util - Add new features, fix history retention bug in prune

* The prune code was not retaining the fine-grained history between
  the last snapshot and current.

* Add a new PFS config variable called 'prune-min' which may be used
  to set the minimum fine-grained history retention, in seconds.  The
  hammer prune code will not prune inbetween any snapshots that are
  within the fine-grained retention period.

* The mirror-stream directive now automatically loops and reconnects
  if the network connection fails.

* The mirror-stream directive now attempts to break-up a large initial
  bulk transfer into smaller transfers, inserting synchronization points
  so a failure in the middle of the large bulk transfer does not require
  starting from scratch again.

* The show directive (show btree) now takes an optional localization[:objid]
  argument (specified in hex).  If specified the directive will search
  the B-Tree for the key, printing nodes as it goes, and then continue
  with a normal iteration.