* mu-store.h, mu-store-read.cc, mu-store-write.cc, mu-store-priv.hh have been reworked
in mu-store.{cc,hh}, it the mix of c/c++ improved
* update all the dependent modules
* make it easier to upgrade an database in place (without user intervention)
* remove the xbatch-size option
Rewrite the contacts-cache backend in c++
Store the contacts as metadata in the xapian database, rather than in a
separate file.
Update the Store to deal with this.