Added stats_pgsql_query_digest and stats_pgsql_query_digest_reset tables #450

Job	Run time
run / test (mysql57, pymysql)	1s
run / test (mysql57, mysqldb)	1s
run / test (mysql57, asyncmy)	1s
run / test (mysql57, mariadbconnector)	1s
run / test (mysql80, pymysql)	1s
run / test (mysql80, mysqldb)	1s
run / test (mysql80, asyncmy)	1s
run / test (mysql80, mariadbconnector)	1s
run / test (mysql84, pymysql)	1s
run / test (mysql84, mysqldb)	1s
run / test (mysql84, asyncmy)	1s
run / test (mysql84, mariadbconnector)	1s
run / test (mysql90, pymysql)	1s
run / test (mysql90, mysqldb)	1s
run / test (mysql90, asyncmy)	1s
run / test (mysql90, mariadbconnector)	1s
run / summarize	1s
	17s

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