* 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
Add an option --lazy-check to ignore any directories that don't have
their ctime changed since the last indexing operation.
There are a few corner-cases (such as editing a message outside mu's
control) where this might miss a change, but apart from that, makes
indexing in for a maildir (and its sub-maildirs) almost a no-op if there
were no changes.