IPLD Resolver: Manage outgoing connections

[aakoshh]

#466 assume that it is possible to open new outgoing connections to peers in subnets we want to resolve content from. There is a limit on incoming connections in #465, so we might have to try a few peers until we find one with free capacity, but we assume we won't run into our own restriction, that if we need to connect to a new subnet and we don't have a connection to it, we'll make room.

For this reason, #465 sets no limit on outgoing connections. We have to make sure though, that the connections opened during peer discovery (unless it's using UDP) and Bitswap don't stay open forever.

One way of achieving the latter is to track in the Content behaviour who we are connected to and close down connections in subnets which are served by too many peers already.