Notifying peers of relay preferences

Currently, block-relay-only connections are established indirectly:

• When making an outbound block-relay-only connection, a node sets the boolean flag fRelay in the version message to false.

• fRelay (introduced in the context of BIP 37) does not imply that transactions cannot be sent for the entire duration of the connection - in its original use case with BIP37, relay of transactions can be activated later on.

• fRelay=false is also used in -blocksonly mode, a low-bandwidth option in which a node does not want to receive transactions from any peer, but does participate in address relay.

Therefore, nodes currently don’t have a notion which of their incoming peers see the connection as block-relay-only and don’t have any logic attached to it.

PR#20726, accompanied by the new BIP proposal BIP 338, introduces the new p2p message disabletx for block-relay-only connections, which makes it explicit that no messages related to transaction relay should ever be exchanged over the duration of the connection.