Propagate inbound socket close event on Peer to the top level P2P class - Closes #3602

[jondubois]

What was the problem?

• The inbound socket close event was not being emitted on top level P2P class.
• We were removing the peer from the peer pool if the outbound socket was disconnected; this is fine for v2.0 but not suitable for v2.1 LIP protocol version since peers will no longer always be both inbound and outbound.

How did I fix it?

• Propagated 'close' event to P2P class.
• For connections state, use constants that are consistent with WebSocket connection states.
• Used strings as enums for connection states for ease of debugging.
• Only remove a peer from the PeerPool if it is disconnected both inbound and outbound.

How to test it?

Run lisk-p2p tests.

Review checklist

• The PR resolves Propagate inbound socket close event on Peer to the top level P2P class #3602
• All new code is covered with unit tests
• All new code was formatted with Prettier
• Linting passes
• Tests pass
• Commit messages follow the commit guidelines
• Documentation has been added/updated