Add a limit for pending messages to a single service

Allows to define the maximum number of pending messages for a single destination service. It extends the existing functionality of checking the number of pending messages from the sender in the message queue. By setting the maximum number of pending messages for a single service can prevent reaching the limit of messages sent by service in case when one of the destination services doesn't respond within the timeout limit, thus preventing the next messages from the sender from being skipped.

Signed-off-by: Marek Maslanka mmaslanka@chromium.org