prevent duplication of candidates with same remote ip:port

[murillo128]

No description provided.

[mildsunrise]

context for anyone who's reading this:

#236 was incomplete because I was only checking candidates added through the JS interface. this PR fixes that and checks candidates learnt by the ICE layer itself, too. but the return also fixes an older memory safety issue that would occur if we called ActivateRemoteCandidate on a connection/transport that does not own the candidate, as the candidate could eventually be removed and this transport would still hold a (dangling) reference to it through its active candidate pointer.

[mildsunrise]

lgtm