The prior system spawned a new connection per request to enable parallelism,
yet kept hitting hyper::IncompleteMessages I couldn't track down. This
attempts to resolve those by a long-lived socket.
Halves the amount of requests per-authenticated RPC call, and accordingly is
likely still better overall.
I don't believe this is resolved yet but this is still worth pushing.
Allows running `cargo build` in monero-serai and message-queue without
erroring, since it'd automatically try to build the binaries which require
additional features.
While we could make those features not optional, it'd increase time to build
and disk space required, which is why the features exist for monero-serai and
message-queue in the first place (since both are frequently used as libs).
