Add a belt-and-suspenders feature, limit the amount of
memory and cpu possible when unlucky or simply misconfigured.
The worst case limit is roughly 400kB * 10,000 = 4GB, regardless
of usage pattern.
Before this change, sheer volume of broadcasts, mismatches in
standardness rules, or simply fee mismatches may result in unbounded
growth of memory usage. As the feature may be expanded in
the future, explicit bounds helps reasoning going forward.
Sending more than one transaction to a given node would be a privacy
leak and thus enforce that this is not done. `GetSendStatusByNode()`
assumes unique node ids.
Note that sending more than one transaction to a given address is fine,
if that is done via separate connections, in which case the node ids
would be different.
Remove the transaction from the list of transactions to broadcast after
we receive it from the network.
Only remove the transaction if it is the same as the one we sent: has
the same wtxid (and it follows the same txid). Don't remove transactions
that have the same txid and different wtxid. Such transactions show that
some of the private broadcast recipients malleated the witness and the
transaction made it back to us. The witness could be either:
* invalid, in which case the transaction will not be accepted in
anybody's pool; or
* valid, in which case either the original or the malleated transaction
will make it to nodes' mempools and eventually be mined. Our response
is to keep broadcasting the original. If the malleated transaction
wins then we will eventually stop broadcasting the original when it
gets stale and gets removed from the "to broadcast" storage cause it
is not acceptable in our mempool.
For connections of type `ConnectionType::PRIVATE_BROADCAST`:
* After receiving VERACK, send a transaction from the list of
transactions for private broadcast and disconnect
* Don't process any messages after VERACK (modulo `GETDATA` and `PONG`)
* Don't send any messages other than the minimum required for the
transaction send - `INV`, `TX`, `PING`.
Extend `PeerManager` with a transaction storage and a new method
`InitiateTxBroadcastPrivate()` which:
* adds a transaction to that storage and
* calls `CConnman::PrivateBroadcast::NumToOpenAdd()` to open dedicated
privacy connections that will pick an entry from the transaction
storage and broadcast it.
