Investigate connection processing speed in HTTP/TCP

**Issue by [drewhk](https://github.com/drewhk)**
_Friday Feb 12, 2016 at 14:54 GMT_
_Originally opened as https://github.com/akka/akka/issues/19765_

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I suspect that the ConnectionSourceStage is the main bottleneck here. This can be decided by:

Take the connection processing code from Http.bindAndHandle and feed it with a source of dummy connection stages that are not TCP connections. This part should be fast enough. If this is indeed fast, then the only bottleneck can be upstream.

Benchmark the TCP connection source and see what speed it can provide. If it is not enough, then the bottleneck has been confirmed, and the stage must be redesigned.

Start at Http.bindAndHandle and inspect the ConnectionSourceStage connection-processing path. Feed the handler with dummy non-TCP connection stages, then benchmark the TCP connection source; the investigation is done when the bottleneck is identified and the need for a stage redesign is supported or ruled out by measurements.

Written by the indexing model from the issue text.