[rpc] Allow a txid param in getrawmempool #10304

See #10300.

utACK 0300d5e556cc1556b9cba1ec11ff3eef47bddca5.

Can you add a test to exercise the new functionality? Perhaps add some calls to getrawmempool with specified txids to the mempool_reorg test. Around here: https://github.com/bitcoin/bitcoin/blob/e2b99b13131b5d0fd6fae6d55892c2ebb327d438/test/functional/mempool_reorg.py#L88

I believe this functionality already exists as getmempoolentry.

@sipa Indeed, I had completely missed that one. I'm surprised there's a whole command for it, personally. (And it doesn't seem to be well known as several people (me, issue author, ..) did not realize it, so it may be an idea to mention it in getrawmempool help.)

Thinking a bit more about it, gettransaction should be generalised to include not only in-wallet transactions, but also in-mempool txs. as we have in-wallet, in-blockchain/non-wallet and in-mempool txs (why entries?). But this is backward incompatible change in the API.

@paveljanik We have getrawtransaction for that, which can query for mempool transactions (and without -txindex, that's pretty much the only thing it is useful for).

I disagree that gettransaction should be extended. Despite its name, gettransaction actually does not return information about a transaction, but is a wallet RPC that returns information about a ledger entry (i.e., the effects a transaction has on your personal balance).