Refactor http codebase

This change-set tries to optimise the way the http related code is organised for better readability and extensibility. It uses streadway/handy as the JSON based logging middleware and in the same fashion implements a metrics middleware to expose prometheus based telemetry.

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/cc @grobie @peterbourgon @bernerdschaefer 
@peterbourgon This might be already covered with test setup like [this](https://github.com/soundcloud/ent/blob/master/ent_test.go#L33-L37). Or you think there should even more isolated cases?