Avoid excessive delays with retries when rpc requests are throttled under disk saturation

I have observed this during code inspection.

RPC server file channel timeout can happen when disk is saturated even with proactive rate control.  On such timeouts, server throttles incoming requests with minimum 30s delay. 
Throttle requests are not bounded by retries as server is active and hence exponential backoff can add significant and unnecessary delays, slowing overall rebalance. Also in addition to  ew Open/Write requests, we throttle older SyncAndClose rpc requests. We can allow the latter since they are the terminal rpc during transfer for somewhat better progress.

Please refer slack discussion: https://couchbase.slack.com/archives/GK1UHGFHU/p1707363622374929