[MySQL engine ] Add hit_count to doclist database index

[BlackbitDevs]

Currently there is this index: https://github.com/teamtnt/tntsearch/blob/a763e66ca1bdebf1fab8f9ed10ec4bdbe2682bb0/src/Engines/MysqlEngine.php#L74

For retrieving all documents for a certain term the following query gets used: https://github.com/teamtnt/tntsearch/blob/a763e66ca1bdebf1fab8f9ed10ec4bdbe2682bb0/src/Engines/MysqlEngine.php#L385

With the current index, this needs to sort the results with filesort:

When we also add hit_count to the index, the filesort is not necessary anymore but filtering and sorting gets fully covered by the index:

This results in better performance.

There is no drawback for other queries which only filter by term_id as MySQL in this case can also use the index prefix (in this case only the column term_id).