The recent changes to mempool inv logic mean that nodes are much less
likely to immediately return an `inv` message in response to a `mempool`
message. The `p2p_txexpiringsoon` RPC test was relying on the prior
behaviour.
`TestNode.sync_with_ping` now takes an optional `waiting_for` closure
that allows the caller to require that a specific message kind is
received prior to the timeout.
The p2p_nu_peer_management and rewind_index RPC tests still start from
Sprout, because they are explicitly (and only) testing network behaviour
across network upgrades.
The mempool_tx_input_limit test is removed because the flag has been
ignored since Sapling activation, and will be removed at some point in
the near future.
When responding to "mempool" message, do not include the txid of an
expiring soon transaction in the "inv" message reply.
When responding to "getdata" message, do not reply with a "tx" message
for a transaction which is expiring soon.