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

[1] is of course id:20181117140901.1870-1-david at tethera.net

>
> (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.
>