Cache refresh performance optimization

Resolves #348

Changing $cachesCleared to static variable, so multiple Tx_Flux_Backend_TceMain::clearCacheCommand requests will not provoke multiple clear refreshes in single runtime.

Tested :
Environment : TYPO3 4.5.30
Flux : current master (87123492789a7c61b0db0b95ace69b8560b778ce)
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