Add cache to target client

[timebertt]

What this PR does / why we need it:
This PR adds a cache to the client for the target cluster. That way, grm's health controller will not retrieve all objects of the manged resource from the API server (by default every minute), but rather use the cache instead. Additionally it only retrieves the resource, if it is of a Kind, that will actually be checked for its health.

Also, grm now does not set .status.resources[].namespace to default for cluster scoped resources (e.g. CustomResourceDefinitions). The previous behaviour seemed to cause problems, when retrieving such resources from the cached client, so I changed it along the way.

Which issue(s) this PR fixes:
Part of gardener/gardener#1953

I did some rudimentary before/after comparison via prometheus in my local setup with the following results:

• Received bytes for

  • grm in garden namespace down by ~65% (~15.5kB/s -> ~5.5kB/s)
  • grm in each shoot namespace down by ~30% (~16.9kB/s -> ~11.8kB/s) (should not really contribute much to external traffic, as most of the managed resources here target the Shoot's API server (in-cluster))

• Sent bytes for

  • grm in garden namespace down by ~41% (~2.81kB/s -> ~1.65kB/s)
  • grm in each shoot namespace down by ~14% (~7.31kB/s -> ~6.31kB/s) (again, should not really contribute much to external traffic, as most of the managed resources here target the Shoot's API server (in-cluster))

Special notes for your reviewer:

Release note:

`gardener-resource-manager` now makes use of a caching client for talking to the targeted API server, which reduces its network traffic.

[rfranzke]

Looks great, thanks! Minor nits.

[timebertt]

/hold
I have one more optimisation I want to make.

[timebertt]

/hold cancel
The optimisation I wanted to make isn't really relevant.

[timuthy]

Very nice :) I just have one suggestion.

[timebertt]

@timuthy and I reworked app.go to not use os.Exit() and return an error instead, and to use a new context and properly stop the manager or the cache when either of them errors.
I also rebased onto master.
PTAL :)

[timuthy]

/lgtm

[timebertt]

Rebased to resolve conflicts.
@rfranzke do you also want to take another look, or can we get this in?

[rfranzke]

Had another rough sketch, but I'm anyways certain that you guys have done it very thoroughly.
/lgtm