btcjson+rpcserver: add a testmempoolaccept

[Roasbeef]

Newer versions of bitcoind have added a new RPC call to allow developers to test mempool acceptance before broadcasting or committing to a given Bitcoin transaction: https://developer.bitcoin.org/reference/rpc/testmempoolaccept.html.

We should also add something similar, as such an endpoint is a good way of exposing all the various policy related checks (though we're no longer 100% in sync with bitcoind's policies). Along the way, we may also want to expose more of the contextless policy checks as public methods on the mempool package, as Go projects can opt to call those methods diectly.