Minimalistic lambda-oriented method for consuming messages

[michaelklishin]

Now that master can require JDK 8 we can begin retrofitting the API to accept lambdas. For example, in some clients it's common to use lambdas as consumer delivery handlers. This is generally straightforward except for one case: cancellation handling and a single lambda
wouldn't work.

I propose that we add a Channel#basicConsume overload that accepts two lambdas instead of a Consumer instance:

• A delivery handler
• A cancellation handler