Since we want to implement a priority queue later, we need to replace
the normal sending channels with the new smart structures first.

UrgentPush allows you to push an rpc packet to the front of the queue so
that it will be popped out fast.

Most importantly, this commit adds a new method called PreValidation to
the interface PubSubRouter, which will be called right before validating
the gossipsub message.

In GossipSubRouter, PreValidation will send the IDONTWANT controll
messages to all the mesh peers of the topics of the received messages.

When receiving IDONTWANTs, the host should remember the message ids
contained in IDONTWANTs using a hash map.

When receiving messages with those ids, it shouldn't forward them to the
peers who already sent the IDONTWANTs.

When the maximum number of IDONTWANTs is reached for any particular
peer, the host should ignore any excessive IDONTWANTs from that peer.

If the messages IDs received from IDONTWANTs are older than 3
heartbeats, they should be removed from the IDONTWANT cache.

Rather than keeping the raw message ids, keep their hashes instead to
save memory and protect again memory DoS attacks.

Co-authored-by: Steven Allen <steven@stebalien.com>

This reverts commit 6fabcdd.

This reverts commit f141e13.