xapcmd: xcpdb supports compaction

xapcmd: xcpdb supports compaction

To minimize the delay on active inboxes, it's actually ideal to
run xapian-compact at the end of the per-partition cpdb process;
since the new DB isn't accessible yet and so we don't have to
deal with lock contention with -mda or -watch processes.  The
downside is temporary file overhead (3x instead of 2x) required.