JSON5 configuration files follow a slightly different syntax and were
silently not being pattern-matched to grep the output file name.

CI jobs still worked because the .dat suffix was hardcoded in the regex,
 and the only .dat files were output files for the current execution.

 The bug became evident now that suffixes are configurable in feature
 branches.

A new templated type-traits structure, `ValueTypeTraits`, is tied to the
getter in `ConfigMap`.

It comprises two statics: the specific Json::Value converter function
(e.g., from Json::Value to uint32) and reflection on the template via a
type name field. These are instantiated in `config.cc` for int, uint32,
and uint64. The default getter is a string, which should trigger no
exceptions.

The functionality that opens a given filename to parse configuration
settings is shifted from Config to closer to its data structure, in
ConfigMap's ctor.

This allows ConfigMap to be parsed in Config's initializer list, in turn
 permitting that its public members be const when needed.

 As a result, getters are not needed in Config, nor is protecting the
 member variables by making them private.

`Json::Value`'s operator[] only accepts string keys, so getters are
still made to take const references to strings.