Simplify `MessageForwarding` entity

So far, `Messageforwarding` has been a trait that requires a
`ExchangeInfo` and a `MessageGateway` provided by its children. It
is used by actors collaborating in the exchange to simplify the
message forwarding.

There is no good reason to use inheritance there. As there is no
good reason to couple the message forwarding to the `ExchangeInfo`
object.

This change turns the trait into a regular class, which receives
the counterpart and the broker connections from its constructor. Now
`MessageForwarding` can be used by composition rather than inheritance.
Merge pull request #169 from apoloval/task/39

Simplify `MessageForwarding` entity