Proxy readiness and make logs messages consistent

Description

Motivation and Context

Closes #145 - removing the need for additional methods in the faas-middleware package, and fixing an underlying bug in the way the readiness endpoint was called in one of the handlers. These other handlers also printed out log messages when using the readiness proxying, so that's been changed, so that it only happens when the User-Agent is not from Kubernetes.

Also: Makes the response messages consistent between HTTP, streaming and serializing modes.

How Has This Been Tested?

Tested with various processes as a Go process on a Linux host.

Including the unit testing added by Lucas in the reviewed PR.

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[x] Breaking change (fix or feature that would cause existing functionality to change)

The log format is changed, if users are relying on parsing this, they can stay on an older version until they can migrate.

However all OF templates use either classic-watchdog or HTTP mode.

openfaas / of-watchdog