Implemented: Custom identifier quoting

Should be sane, even limited compared to what zetacomponents offer. But
with the limited set of table and column names, this should be fast
enough.
Use in memory cache for content types

Only requests in one single request.
Fixed type hint
Re-enabled language cache
Use eq() instead of like() for int comparisons
Added: Additional indexes
Fixed group annotation
Implemented: Cache clearing

Especially mind: We need to clear internal caches during rollback.
Clear internal caches when retrieving repo
Extract method

Made code readable again and removed unused bits
Fixed: unit tests not correctly written
Fixed: integration tests not correctly written
Docs and more unification
Docs
Removed wrong type hints
Removed more overly complex getIdentifier calls
Documented optimization hacks as such
Use join for subtree criterion queries
Changed: The way to generate unique table name alias
Changed: applying index changes to other DB implementations