Revert to events_end terminology in client sync API; update semantics.

Instead of returning events in the interval [events_start, next_event), return
events in the interval (events_start, events_end]. This means that clients can
poll on /sync/events?stamp=<events_end value> to check for new events.
Add imaplib.IMAP4.abort to exception classes for retry_crispin.

This is so that we can retry on imaplib.IMAP4.abort('socket error: EOF')
errors.