Prevent AccountSynchronizier from deadlocking under certain conditions

The AccountSynchronizer could deadlock waiting for an EmailPrefetcher
to finish under certain conditions, particularly if the folder closed
before the prefetcher was finished.  This lead to a state where the
AccountSynchronizer would never synchronize against unopened folders.
If Geary was left open for a long time and the user clicked on All
Mail, rather than the current state of that folder appearing
immediately, it could take a moment for Geary to fetch the envelopes
and headers/bodies.

Fixing this closes bgo#713447, which was also due to other bugs that
have been fixed over time (in particular, the startup speedup work in
0.5) and better spinner management (bgo#713703).  It also corrects
a problem where detection of new mail while opening the folder let
to prefetching that mail, which was unnecessary because once the
folder is fully opened all unsynchronized mail is detected and pulled
down.