WIP2: index user headers

[David Bremner]

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.