transport-manager: Race between dialing and canceling

[lexnv]

In the transport manager we might end up with the following race-condition:

T0. we dial peer X on multiple protocols (State::Opening - outbound connection)
T1. peer X established inbound connection with us

T2. a future from T0 finishes and we'll receive the event at the next poll
T3. we cancel the opening futures started at T0

The impact of the race condition is that we will accept the established outbound connection if we have space for a secondary connection. This is happening regardless of the opening future being canceled, it might be completed before cancellation.

This is similar to the mem leak: #272