Xapian supports an "ngrams" option to help with languages/scripts
without explicit wordbreaks, such as Chinese / Japanese / Korean.
Add some plumbing for supporting this in mu as well. Experimental for
now.
The default was 250000 but that led to problems on some systems with
limited memory, esp. since mu's indexing does quite a bit more than in
the olden days (e.g. html mail).
Fixes#2529.
XapianDb is a fairly thing wrapper around Xapian, which handles locking,
exception handling and some tracking.
On top of that, Config add a configuration database for type /
introspectable configuration info.
Dirk-Jan C. Binnema