Make resend timeout configurable

Previously, the general receiver timeout was used for the time to wait before resending unacknowledged message. This time is usually under 5s which is too short for this purpose.

I am not sure what is a good default. Too short, and messages will be resent unnecessarily while the original message is still being processed, too long and the message might already be dropped from the memory buffer and cannot be resent if not persisted.

Closed #11 (closed)