Support cases where the build directory is not the mapcache root

Cmake is commonly built from a `build` directory in the package
root. This (and any other esoteric build location) is now supported.
Add code coverage analysis, wrapping up development in a Makefile
Module cleanup is called by `atexit()` rather than `Node::AtExit()`

There is no node or V8 specific code in the cleanup so it can be
called by `atexit()`.
Improved test coverage
Removed the `httpCacheHandler` API function

This was removed because:

- It is an undocumented feature.

- The examples show how to use the cache in combination with the
  `http` module.

- Connecting the cache to the `http` module is a bespoke business.
Updated coverage stats