p2p: Announce reconciliation support

If we're connecting to the peer which might support
transaction reconciliation, we announce we want to reconcile
with them.

We store the reconciliation salt so that when the peer
responds with their salt, we are able to compute the
full reconciliation salt.

This behavior is enabled with a CLI flag.

src/test/fuzz/process_message.cpp

@@ -130,6 +130,7 @@ FUZZ_TARGET_MSG(pong);
 FUZZ_TARGET_MSG(sendaddrv2);
 FUZZ_TARGET_MSG(sendcmpct);
 FUZZ_TARGET_MSG(sendheaders);
+FUZZ_TARGET_MSG(sendtxrcncl);
 FUZZ_TARGET_MSG(tx);
 FUZZ_TARGET_MSG(verack);
 FUZZ_TARGET_MSG(version);