Opening a notmuch DB gives you a snapshot of the stage at that specific time.
Prior to this, we only reopened the DB upon writing.
However, if say a mail sync program like offlineimap is fetching new mail,
we would never pick it up.

This commit caches a db for a while, so that we don't generate too much overhead
and does a reconnect cycle after that.

I hardcoded a value as I don't think that having an option would be beneficial.
Any write operation (meaning reading mail) anyhow flushes the DB by necessity.
(we need to close to commit tag changes, which changing the read state is)

Exposes the notmuch tags accordingly, stubs it for the maildir worker.

Right now notmuch panics if something goes wrong in the configure event.
This patch checks for that and returns an error instead, so that we can at least
get the UI up and running (and all the other accounts)

The experience will be completely degraded until another configure event occurs.