Deduce a timezone for tasks due and start date

Summary:
Activesync doesn't know about timezones, but submits a utc and non-utc
value. We attempt to preserve this information by matching a timezone
with the same offset, so we can can replay a task with the same values
to another client.
If we don't do this the time will be stored as utc, and we will end up
replaying utcDueDate == dueDate, which will result in the client
displaying the task with an offset of the utc offset.
This solution will break down with clients in different timezones, but
that's just how it is unless we find a way to detect the clients
timezone.

Differential Revision: https://git.kolab.org/D1864