Huge regression in case of too many HTTP requests

[librelois]

I tested the http_concurrent_conn_calls benchmark on a machine with 32 cores, and I see indeed a huge regression with jsonrpsee, about 98%!

I ran it in this order:

1. jsonrpsee master 78055fe
2. jsonrpc

In 2. I observe a performance gain of 98% in sync/512, sync/1024, async/512 and async/1024, similar performance in the other cases.

This means that jsonrpsee is twice as slow as jsonrpc in cases sync/512, sync/1024, async/512 and async/1024.

[niklasad1]

Closed by #718, feel to try it out again @librelois

A followup PR is coming soon where it will possible to configure to backlog in the HTTP server.