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.
