AsyncTerminatedMessageStream: limit message count

Currently the message stream can process any number of messages in
parallel (if they get dispatched to different threads or processes).
In order to limit their number we only handle messages and read from
the socket if we're under a certain limit of unanswered ones.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>