local rpc: new command: "inspect_session"

[SomberNight]

example:

$ ./electrumx_rpc inspect_session 4
{
    "client_name": "electrum/4.7.2",
    "conn_time": "38s",
    "errors": 0,
    "is_logged": false,
    "pending requests": 0,
    "request counts": {
        "blockchain.estimatefee": 6,
        "blockchain.headers.subscribe": 1,
        "blockchain.scripthash.subscribe": 66,
        "mempool.get_fee_histogram": 1,
        "mempool.get_info": 1,
        "server.banner": 1,
        "server.donation_address": 1,
        "server.features": 1,
        "server.peers.subscribe": 1,
        "server.ping": 1,
        "server.version": 1
    },
    "request total": 81,
    "subs_sh": 66,
    "subs_txo": 0,
    "txs sent": 0
}