Aggressively update in-memory version numbers

A large version number to issue is stored on disk, and a copy is cached in
memory.  This patch aggressively updates that copy on writes.  The practical
upshot should be that checkpoints converge significantly faster than they would
otherwise.
Fix a segfault in previous commit (caught by testing).

For spaces with subspaces, or when performing transfers, it assumed an entry
would be in a map when, in fact, it was not.  This would lead to a NULL ptr
dereference.  Caught by the replication-stress-test.
Update YCSB to not abort on reconfigure
Expose a subset of transfers at a time
Ensure daemons always finish checkpoints.

Under heavy reconfiguration activity, a daemon could drop a checkpoint until
either reconfiguration (again), or a USR2 debug signal was sent.
A bunch of fixes.

 - Expose transfers using a stable sort, so once exposed a transfer will forever
   remain exposed.
 - Issue a new configuration if checkpoints seem stuck (should never happen
   anymore, but this is a safety measure just in case).
Upgrade between 1.3.x and 1.4.0
Ensure that "loop" always maintains the coordinator connection or otherwise makes progress