Adds generators support for email backends that do not support it.
Fixed 20613 - moved pickle loads/dumps outside with self._lock.reader()/writer() blocks
Additionaly added pickled=None initially and check for none before
returning in get
Removed some logic that wasn't needed following removal of some deprecated code.
.DS_Store can be ignored.
Merge branch 'ticket_20613' of https://github.com/ersran9/django into t20613
A few cleanups to capture the minimum amount of code in try/except blocks.
Fixed #13813 -- Comparison of DatabaseWrappers doesn't raise errors.

Patch from Lukasz Balcerzak.
Merge pull request #991 from stockr-labs/feature/email-backends-generators

Fixed #20194 -- Adds generators support for email backends that do not support it.
Replaced some dicts with sets.
Fixed a bug I introduced in my previosu ommit.
Tests passed when given patch was applied.
t Revert "Tests passed when given patch was applied."

This reverts commit 8ab9c2b26f024436193d3900cc0f195f9641a1f5.