improve mgcp client API
The API of the mgcp client is not very optimal designed and we should change it. The problem is that the set of functions that return the generated MGCP message take all parameters separately. MGCP messages have plenty of parameters and when we need one more parameter we have to add another function. I already did this a few times now.
The solution to the problem would be just one (or at least one per message type) generator function that takes a struct. Depending on how the struct is populated the parameter will be included in the MGCP message or not.