Previously only files specified in the ConfigMap were checked for
changed modification times. If only generated VCL sources were
changed, usually due to Endpoint changes, and the VCL load failed
for any reason, tehn afterward no changed VCL sources or changes
in the ConfigMap were detected. This could lead to VCL never being
updated after a change only in the generated VCLs, although the
config does not correspond to the desired state of the cluster.

Now we keep a record of the latest mtime of all VCL sources,
including generated sources, and reload VCL if any of them are
currently newer.

Types for literal structs don't need to be named when they are
unambiguous.

At info level, log the latest known VCL mtime. Log all of the mtimes
at debug level.

This may happen if the initial cache sync takes longer than the
monitor interval.

Take no action for a deleted ConfigMap, other than resetting current
knowledge of VCL mtimes.

This is a key in the ConfigMap used for VCL configuration. If set,
and if the value is not empty, then the value is a version string
for the VCL config. It is used in the generation of the VCL config
name, and in the fqVersion function for VCL templates.

Eliminates a warning message at startup.

After vcl_cooldown, VCLs since Varnish at least 6.3 go to cold
temperature, but stay in the auto state.

Cold state happens when a VCL is set to cold via vcl.state, in which
case the temperature also goes cold. So an admin might hasten the
discard by setting the cold state.

It should be configurable whether this happens -- an admin may want
to intentionally stop Varnish, and this feature would interfere with
that.

Get some mod tidy done while we're here.

In case Pod names ever change without changing endpoint addresses.

We don't need a binary-to-text encoding for the hash, we just test
the binary result. We also don't need to sort []Endpoint or hash
Service.

InitTemplates() takes a map of template paths to file paths, and
returns a map of parsed template to file paths.

Add WriteTemplate(), which takes a parsed template and a file path,
and generates VCL at the path from a spec and the template. Meant
for iteration on the map returned by InitTemplates().