api: Add all in-flight requests /reverse_proxy/upstreams (#7277)

[u5surf]

Assistance Disclosure

No AI was used.

Fixes: #7277

Introducing new usagePool named inflightHosts which store the host is still in-flight upstream in reverse proxy Cleanup phase. and when proxyLoopIteration returns done, the inflightHost drain in the usagePool.
Here is scope for improvement because it is draft status now.

[u5surf]

We would like to Get by key methods simply sounds like this instead Range

val := inflightHosts.Get(reqHost)
host, ok := val.(*Host)
...
if host.NumRequests() == 0 {
	inflightHosts.Delete(reqHost)
}