prevent notifications from firing until the queue is empty on startup/reconnect

[Trolldemorted]

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As has been discussed in #98, we should wait until we have drained the queue before we fire unneccessary notifications.

Requires libsignal support, but the changes should be minor.

[Trolldemorted]

libsignal is now supporting this.

OnMessage is now called with a SignalServiceMessagePipeMessage, which may be a SignalServiceMessagePipeEmptyMessage.