After sending, a client might append to the sent folder over IMAP.
In this case, we perform deduplication and return the message ID of the
sent message. However, if we haven't already processed this message in
gluon, it doesn't work as expected.
This change polls the event stream immediately after send. Note that it
doesn't wait for these events to be processed; that should be done in a
follow-up commit.
a213b48f93