WIP2: index user headers

[David Bremner]

David Bremner <david at tethera.net> writes:

> This obsoletes [1]
> This is getting closer to mergable, but it still needs at least to
> sanity check the names of user defined prefixes (see point (a) below).
>
> The main differences from [1] are
>
> (a) xapian prefixes are no longer defined via upper casing, as this is
>     locale dependent. The do rely on a ":" separator, hence the need
>     for some sanitization.
>
> (b) The caching of user header/prefix information is now done via
>     string maps, and used more effectively during indexing.

I had another thought about user prefixes. I wonder if they should all
be forcibly prefixed with something that prevents collisions, to prevent
later pain if we add an "official" prefix with the same name. A quick
tests suggest it would work to use something like _

so

        notmuch search --output=files _list:notmuch

works. It's a bit ugly, I'll have to play with other options; the main
question is whether we think prefixing is needed / worth-it.