Address possible blocking calls in forwarders

[pmalek]

As of now the forwarders don't use a context.Context as an argument to Forward().

This might cause issues like blocking other pieces of code whenever the consumer of the report being forwarded do not respond in a timely manner.

This could also be a problem in implementations like rawChannelForwarder where the consumer is responsible for consumption of the report in timely manner and if that's not the case then the whole pipeline can stall.

Related comment from a review: #45 (comment)