Since https://github.com/kubernetes/kubernetes/pull/112648, we can
efficiently handle selectors from pre-existing `map[string]string`,
making the cache obsolete.
Benchmark:
```
name old time/op new time/op delta
GetPodServiceMemberships-48 189µs ± 1% 193µs ± 1% +2.10% (p=0.000 n=10+10)
name old alloc/op new alloc/op delta
GetPodServiceMemberships-48 59.0kB ± 0% 58.9kB ± 0% -0.09% (p=0.000 n=9+9)
name old allocs/op new allocs/op delta
GetPodServiceMemberships-48 1.02k ± 0% 1.02k ± 0% ~ (all equal)
```
The field is not used anywhere and its value may be stale as Endpoints
and EndpointSlice won't be updated if there is only Pod ResourceVersion
change..
When comparing EndpointSubsets and Endpoints, we ignore the difference
in ResourceVersion of Pod to avoid unnecessary updates caused by Pod
updates that we don't care, e.g. annotation update.
Otherwise periodic Service resync would intensively update Endpoints or
EndpointSlice whose Pods have irrelevant change between two resyncs,
leading to delay in processing newly created Services. In a scale
cluster with thousands of such Endpoints, we observed 2 minutes of
delay when the resync happens.
The endpoint controllers responded to Pod changes by trying to figure
out if the generated endpoint resource would change, rather than just
checking if the Pod had changed, but since the set of Pod fields that
need to be checked depend on the Service and Node as well, the code
ended up only checking for a subset of the changes it should have.
In particular, EndpointSliceController ended up only looking at IPv4
Pod IPs when processing Pod update events, so when a Pod went from
having no IP to having only an IPv6 IP, EndpointSliceController would
think it hadn't changed.
This was an oversight in the initial EndpointSlice release. This update
will ensure that Endpoints and EndpointSlices use the same logic to set
the Hostname attribute.
