WIP2: index user headers
david at tethera.net
Sat Mar 2 07:41:26 PST 2019
This obsoletes 
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  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.
More information about the notmuch