Fixed retry without backpressure & test function to support bp.

Fixes include:

- do not decrement the client capacity if it is unbounded,
- do not request 0,
- propagate request count (n) instead of capacity + request (c + n),
- FuncWithErrors to support backpressure; there was a non-zero chance the final succeding pair of onNexts would trigger a MissingBackpressureException.