[TASK] Replace static property caches in RootlineUtility

RootlineUtility uses a multi-staged cacheing logic. This
contains the use of a dedicated rootline-cache, but also
two static class internal caches. Static property caches
are a horrable nightmare when it comes to keep proper
clean state between test executions, beside it's a not
really needed state on a non-static class.

This change aims to replace the static property caches
in RootlineUtility with the core runtime cache. Thus
making the static properties obsolete, but keeping it
until testing-framework can deal with the absence of
the static properties. Remove them!

`RootlineUtility::purgeCaches()` calls are replaced now
with core runtime cache flush calls using tags to avoid
flushing other unrelated runtime cache entries. This makes
this method obsolete. This method was declared @internal,
eliminate it!

Additionally, default `$rootlineField` merging with the
`$GLOBALS['TYPO3_CONF_VARS']['FE']['addRootLineFields']`
is moved from RootlineUtility ...