Reduce database lags: Bug #725929

This reduces lags by searching for incomplete emails in slices as
well as in separate database transactions.

This is a refinement of commit 6672f8, where the database was
searched in slices but in one transaction.  This didn't completely
solve the db lags in this code path.
Merge NEWS and debian/changelog from geary-0.6 branch
Updated to 0.7.2
Merge https://git.gnome.org/browse/geary