Invalid response completes transaction

[mickel8]

IDs of transactions that are in-progress are stored in client structure. When STUN success/error response arrives, we first look for the request this response relates to, then we pop the request from the client structure, and process the response.

If we receive an invalid response, which should be ignored (e.g. a response with incorrect message integrity, missing attributes etc.), we will pop the request from the client structure too. When the correct response arrives, we will ignore it as there is no request we await a response for.