Fix composed email Date: header: Closes bug #714376

GMime documentation suggests its date conversion methods take a
time zone offset in hours, but it appears the number must be base-100
to get the right results (i.e. -7 becomes "-0700").
Some date-related fixes
Stop progress monitor when open has completed: Closes bug #728936

This ensures the progress monitor has stopped when opening has
completed, whether successful or due to error.  Because the
remote_open_async() call can be reentered if a connection re-
establishment is attempted, important to stop the monitor before
calling close_internal_async().
Improved error detection and retry during folder open
Merge branch 'master' into wip/only-incomplete
Enable concurrency in Db