daemon: unconditionally close Xapian shards on cleanup

daemon: unconditionally close Xapian shards on cleanup

The cost of opening a Xapian DB (even with shards) isn't high,
so save some FDs and just close it.  We hit Xapian far less than
over.sqlite3 and we discard the MSet ASAP even when streaming
large responses.

This simplifies our code a bit and hopefully helps reduce
fragmentation by increasing mortality of late allocations.