Update pop3 syncing

b/9857766
If we currently have zero messages (i.e. initial sync of
a new account) we will load the most recent 100 messages.
On subsequent syncs, we will load more recent messages
until either:
1. We have loaded 100 and not yet past any of the messages
we already had locally, or
2. We encounter messages that we already had locally, and
then load "deltaMessageCount" additional messages past
the ones we have locally.

Change-Id: I8b34409dcdaceff2d2e1906174241a74f2331688
1 file changed