[ipfs/go-bitswap] Remove unresponsive peers from Sessions

[dirkmc]

If a peer doesn't respond to any request for some time, we should remove the peer from all Sessions.

Proposal:

• For each peer, PeerManager keeps track of
  • the last time a want was sent to the peer
  • the last time the peer responded with HAVE / DONT_HAVE / block
• PeerManager has a ticker that periodically checks each peer to see if the delta exceeds a timeout
  • if so it signals to all sessions that are interested in the peer that the peer is no longer available
• if a Session receives DONT_HAVE from all available peers it will broadcast want-have to all connected peers (including those that are not in the Session). If the peer is alive it may respond and be added back into the Session.

Depends on ipfs/go-bitswap#259