SFC changes

Various changes described in the subtasks:
1. Your FC2 is essentially SFC plus (a) credits replenishments only return the bytes read not the full credits (OK), (b) they are sent before available credits have been exhausted and (c) with unicast flow control.
2. (a) makes a lot of sense, I'll add it to SFC. (BTW: FC used to do this, but then I changed it to make it simpler )
3. (b) Not sure this makes sense; this can be achieved (e.g. in FC) by setting max_credits to a smaller value and increasing min_threshold. I agree that sending smaller number of credits in smaller interval is better, I measured this in my perf tests with SFC. OK, I'll likely implement (b) in SFC as well
4. (c) The reason I didn't do unicast flow control was that SFC was simpler than FC, and I thought we don't need unicast flow control as there are not many unicast messages. Enter Buddy Replication with anycast, and that argument is flawed...