Add batch consume method to consumer

[tburmeister]

This still needs some work, but wanted to get thoughts on the general idea. Goal here is to add a batch consume method, already available in the C and Python clients, for more efficient consumption of messages. I've included an example of usage, as well as two examples for benchmarking the batch consumer against the current Poll method. In my benchmarks, Saw the following speedups for consuming 1M x 512B messages from a local Kafka server:

poll:
real    0m4.138s
user    0m1.471s
sys    0m0.307s

batch consume:
real    0m0.721s
user    0m0.871s
sys    0m0.208s

There are currently two pretty big limitations to my implementation:

1. You must turn off auto offsets commits;
2. You must use the Assign method to manually assign topic-partitions; you can't use the high level Subscribe method.

Basically, the commit and assignment callbacks cause a panic.

[ghost]

@confluentinc It looks like @tburmeister just signed our Contributor License Agreement. :+1:

Always at your service,

clabot

[lexasoft123]

@edenhill might be this PR can resolve CGO bottleneck? :)

[syepes]

Is this still in active work? it would be nice to also have this on the Go client

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[jflambert]

Any updates @tburmeister ?