And use the fake interface in the unit tests, removing the dependency
on setting up FakeExec stuff when conntrack cleanup will be invoked.
Also, remove the isIPv6 argument to CleanStaleEntries, because it can
be inferred from the other args.
Add an interface between CleanStaleEntries and the lower-level
conntrack helpers (ClearEntriesForIP, etc), and a fake implementation
of that interface, so that we can explicitly test CleanStaleEntries's
logic.
Remove some comments from conntrack.go that were explaining the
functions' callers rather than explaining the functions themselves
(and which were redundant with other comments in the callers anyway).
The use of "Endpoint" vs "Endpoints" in these type names is tricky
because it doesn't always make sense to use the same singular/plural
convention as the corresonding service-related types, since often the
service-related type is referring to a single service while the
endpoint-related type is referring to multiple endpoint IPs.
The "endpointsInfo" types in the iptables and winkernel proxiers are
now "endpointInfo" because they describe a single endpoint IP (and
wrap proxy.BaseEndpointInfo).
"UpdateEndpointMapResult" is now "UpdateEndpointsMapResult", because
it is the result of EndpointsMap.Update (and it's clearly correct for
EndpointsMap to have plural "Endpoints" because it's a map to an array
of proxy.Endpoint objects.)
"EndpointChangeTracker" is now "EndpointsChangeTracker" because it
tracks changes to the full set of endpoints for a particular service
(and the new name matches the existing "endpointsChange" type and
"Proxier.endpointsChanges" fields.)
