* Added FileRepo::SKIP_LOCKING constant and made storeBatch() check it.

* Added FileRepo::SKIP_LOCKING constant and made storeBatch() check it.
* Made File::maybeDoTransform() use the FileRepo::store() and a new File::getThumbRel() function. Looks cleaner and loosens FileBackend coupling.
* Also made storeTemp() use FileRepo::SKIP_LOCKING for performance.
* Killed some useless initZones() calls in FileRepo. Extensions may not even use these zones. Likewise, it could make tests fail even though they don't those zones. We already do the sanity with some prepare() calls in storeBatch().
* Removed FileRepo::SKIP_VALIDATION, not used by anything now.
* Moved getUrlRel() down a bit.