Modified statistics schema structure + store the statistics in the

statistics/DatabaseStatisticsService.cpp

statistics/DatabaseStatisticsService.hpp

statistics/FileStatistics.cpp

statistics/FileStatistics.hpp

statistics/MySQLStatisticsService.cpp

statistics/MySQLStatisticsService.hpp

statistics/Statistics.cpp

statistics/Statistics.hpp

statistics/StatisticsSaveCmd.cpp

statistics/StatisticsService.hpp

statistics/StatisticsUpdateCmd.cpp

statistics/common_statistics_schema.sql