This constant is set to 2 minutes, rather than 15. This is still many times
larger than the transaction broadcast interval (2s for outbound, 5s for
inbound), so it should be acceptable for peers to know what our contents of
the mempool was that long ago.
This is a partial backport of [[https://github.com/bitcoin/bitcoin/pull/19109 | core#19109]] [2/5]
https://github.com/bitcoin/bitcoin/pull/19109/commits/b24a17f03982c9cd8fd6ec665b16e022374c96f0