notmuch: Switch from gmime to custom, ad-hoc parsing of headers.

Since we're currently just trying to stitch together In-Reply-To
and References headers we don't need that much sophistication.
It's when we later add full-text searching that GMime will be
useful.

So for now, even though my own code here is surely very buggy
compared to GMime it's also a lot faster. And speed is what
we're after for the initial index creation.