Stop/minimise mu database locking

[Chris Warburton]

The inboxen service runs mu index to ensure new messages are picked up.
This will also pick up new news articles (inboxen runs more often, so we
put the indexing in there).

This can make mu4e unable to open messages for a time. This is annoying,
so we should find a way around it (either avoiding the locking, reducing
the time spent, allowing both to access, etc.).