Set custom query for count
#5505 - constant usage instead of magic constants
#5505 - removing unused import
#5505 - removing useless `= null` default property assignments
#5505 - uppercase `ROW_COUNT_COLUMN_NAME` for DB2/Firebird compat
#5505 - Test case cleanups (docblocks, setup logic), assuming 4th constructor argument instead of `DbSelect#setSelectCount()`
#5505 - Additional constructor argument instead of additional setter
Merge branch 'feature/#5518-allow-custom-row-count-column-name' into develop

Close #5518