Fixes  https://github.com/piwik/piwik/pull/6652

Add unit tests for new Metrics Formatter classes and remove existing ones. Small change to Overlay included and bug fixes to metrics formatting code included.

Declares a process as finished as soon as a PID file is > 500bytes
(contains only PID) and output file is > 100MB (which should usually
not be larger than 100KB or 1MB) to prevent files growing up to many
GBs. I added unit and integration tests for the file size detection. It is
now possible to mock the methods file_exists and filesize although very
simple so far. Later we can allow to define callbacks or to define different
return values for different files or we can use something like vfsStream

The `tmp/` path was hardcoded everywhere, which resulted in using `SettingsPiwik::rewriteTmpPathWithInstanceId()` to rewrite it for specific use cases.

I've moved that path into the config, and replaced all hardcoded usage (and calls to `rewriteTmpPathWithInstanceId()`) by a `get()` from the container.

Getting entries from the container is a bad practice and dependency injection should be preferred, but we do baby steps. When refactoring those classes to DI, we'll replace calls to the container with proper dependency injection.
Another thing we'll need to do too is move the hardcoded *sub-path* of `tmp/` (e.g. `tmp/sessions/`) into the config also (but again: baby steps).

Another future step would be to remove completely instance ID and let it be handled by a plugin (or by end-user config). Having the `tmp/` path in the config means that plugins or users can override it and know it will be taken into account everywhere in Piwik.

Moved processed metrics computation to DataTableGenericFilter, removed new filters & implemented as methods in Report to avoid confusion regarding re-use, allow adding processed metrics as DataTable metadata and use this to rewrite AddProcessedMetrics filter, correct name of Metrics::getMappingFromIdToName function, added placeholder AggregatedMetric class for future, revise Metric/ProcessedMetric hierarchy (add methods for translation/etc.), allow .get API methods to use metadata to automatically figure out which columns to select, get EcommerceOrderWithItemsTest to pass.

refs #6481 in report generator always show the dimensions of the selected plugin even if the plugin is not enabled

Fixed getCurrentUrl() and getCurrentUrlWithoutQueryString() + tests