Add helper to receive last message

[Shatur]

When you working with unreliable sequenced channel you sometimes want to receive only the last message. Currently I have to call receive_message manually until it returns false which is not very ergonomic.

[lucaspoffo]

There is no option for sequenced channels anymore, so this auxiliary function doesn't make a lot of sense now.

For unreliable sequenced messages, the user must implement it. This could be done by adding a sequenced_id to the unreliable messages and keeping track of it - dropping messages below the sequenced value.