buffet: When XMPP channel created, poll the server for commands once

As soon as we get confirmation that XMPP channel is established, we will
rely on push notification for command delivery from then on. However we
need to make sure there have been no new commands queued since the last
time we polled the server and before XMPPP channel was created.

So, poll the server once more after we have established XMPP channel,
to make sure there is no command pending.

TEST=`FEATURES=test emerge-storm buffet`

Change-Id: I496917d96ae5c9cae1b9c7715f6b2b39c3068606
Reviewed-on: https://chromium-review.googlesource.com/285921
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Aaron Kemp <kemp@google.com>
Tested-by: Aaron Kemp <kemp@google.com>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
2 files changed