* Do not inherit LocalStatusSQLiteFolder class from the plaintext
one.
* Use some functions already in BaseFolder in both, plaintext and
sqlite classes.
* Add a saveall method. The idea is that saveall dumps the entire
messagelist to disk, while save only commits the uncommited
changes. Right now, save is noop for sqlite, and equivalent to
saveall for plaintext, but it enables to be more clever on when we
commit to disk in the future.
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>