Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>

Introduces a single-flight call to a function's health
endpoint to verify that it is registered with an Istio
sidecar (Envoy) before letting the invocation through.

Results are cached for 5 seconds, before a probe is
required again.

Tested without Istio, with probe_functions environment
variable set to true, I saw a probe execute in the logs.

Fixes: #1721 for Istio users.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>

Enables publishing to various topics according to annotations
on the functions. The function cache is moved up one level so
that it can be shared between the scale from zero code and the
queue proxy.

Unit tests added for new internal methods.

Tested e2e with arkade and the newest queue-worker and RC
gateway image with two queues and an annotation on one of the
functions of com.openfaas.queue. It worked as expected including
with multiple namespace support.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>