monero-simple-request-rpc is inefficient

[kayabaNerve]

For each request, a dedicated connection is used. While there is internally a connection pool, connections should be reused at a higher level via JSON-RPC request/response IDs.

Low priority, yet would be important to anyone who wants a fast RPC. This isn't a breaking API change if monero-simple-request-transport manages the IDs. If we were to have monero-rpc do so, it would be (as the transport trait would need to be send request, yield responses).