Update_Batch

Compiles an update string and runs the query

@param array   $set       An associative array of update values
@param string  $index     The where key
@param integer $batchSize The size of the batch to run
@param boolean $returnSQL True means SQL is returned, false will execute the query

@return mixed    Number of rows affected, SQL string, or FALSE on failure
@throws \CodeIgniter\Database\Exceptions\DatabaseException