First draft of logging functionality.

[david at tethera.net]

The patches following this message are my first attempt at
implementing atomic logging for notmuch.  The idea is that such logs
could be useful in synchronizing notmuch instances.

Feedback of any kind is welcome. I'm particularly interested in
comments about the log format and performance.

In my tests, logging adds about a 10% speed penalty (tagging ~3700
messages) if enabled. I'd be curious if people for whom tagging is
slow could tell me if they take a bigger hit. If you want to test, add
a stanza like

[log]
tags = /path/to/logfile

to your notmuch config.

About the format, I am currently using

seconds-since-epoch json-quoted-message-id json-quoted-plus-minus-tag

I know some of you will not be thrilled with the quoting format; I'm
open to other ideas, but this is what was already existing in notmuch
code.  At the moment I wanted to push the quoting fairly far down the
stack and use the same for all potential logging, but perhaps this is
too much "design by convenient implementation".

Canadian-Thanksgiving-Greetings,

David