transport/manager: Determine if remote peer can establish two connections with local node

[lexnv]

Currently the transport manager can accept two connections established by the remote, however the local node can establish only one connection.

This comes from the fact that each transport will eagerly negotiate the incoming connections and then report to the transport manager that the connection has been established. Then the transport manager accepts the connection if there's a secondary slot.

Determine if the litep2p node should accept 2 incoming connections, instead of 1 incoming and 1 outgoing in the rare cases where we dial the peer and at the same time the peer connected to us. From the resource perspective, rejecting a second incoming connection would be beneficial, and would lean towards this.

Followup from: #185 (comment)