Previously, we would potentially add the same txid many times to the rolling
bloom filter of recently announced transactions to a peer, if many outputs of
the same txid appeared as inputs in a transaction. Eliminate this problem and
avoid redundant lookups by asking the mempool for the unique parents of a
requested transaction.
This is a backport of core#19596 [1/2]
https://github.com/bitcoin/bitcoin/pull/19596/commits/81961762439fb72fc2ef168164689ddc29d7ef94