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.

A segmentation fault occurs when using the notmuch backend and a `query-map`
file that contains blank lines or comments.

Syncs back special notmuch tag like unread to the underlying maildir store

For some reason the current code frequently segfaults due to an
invalid C memory address. This commit mediates that by never keeping an object
alive longer than absolutely necessary.

Notmuch only allows a single write connection, all other clients trying to
modify the db block. Hence we should only open one when we actually need it.

Apparently we also need to refresh the RO DB connection upon modification,
else we get stale message tag results

This commit introduces the notmuch backend.
The backend is conditionally compiled in if the "notmuch" tag is provided.

Most of the message types are implemented, with the notable exceptions
of DeleteMessages as well as any copy / move / append type.
Reason being, that those aren't normally applicable in a notmuch based workflow.

Changes v2 --> v3, based on review comments
* Use account config for configuration