Add some usleeps for indexing scheduler tasks

[rengaw83]

To relax the server the tx_mksearch_service_internal_Index::executeQueueData method has to perform a usleep of ~250 ms after each index and ~10 ms after each queue record.
This is useful for a high frequented page or a TYPO3 system on a shared hosting with limited cpu power.

[hannesbochmann]

It should be a feature which can be activated directly in the scheduler job. The sleep intervalls should be configurable as well.