Sending control extension message to remote peers

[lucassaldanha]

The overall logic for sending control extension messages to remote peers is simple:

• always report support for all extensions (finer-grained control to be implemented as part of Feature flag for supported extensions #441)
• control extension message is sent whenever a new peer connects or when we are subscribing to a topic (to handle incoming and outgoing connections)
• GossipExtensionsState class was added to GossipRouter to help keep track of which peers our node has already sent a control extension message to, to avoid sending it again.
• When a peer disconnects, we clear the information associated with that peer from GossipExtensionsState
• Control extensions messages are only sent when the gossipsub protocol version supports it (1.3+)

part of #436

[StefanBratanov]

LGTM