Since message IDs are now static but there's no good way to generate
static thread IDs while syncing an account from newest message first,
we give threads the ID of any message on that thread and, when setting
metadata, look up the local thread ID by first going through the
message table.
This will only generate 1 delta per new message instead of 2
Adds a MessageFactory to create messages from imapmessage objects, and
unifies and cleans up that logic
Also modified the message-processor to be more suited for local sync,
made emphasis that it's only for processing new messages, and got rid of
the while "pipeline" concept
- TODO message-processor specs are broken
