add clear method in InMemoryStore #4425

Job	Run time
mariadb (locked, 8.3, ubuntu-latest, 10.5)	51s
mariadb (locked, 8.3, ubuntu-latest, 10.6)	1m 25s
mariadb (locked, 8.3, ubuntu-latest, 10.11)	52s
mariadb (locked, 8.3, ubuntu-latest, 11.1)	1m 18s
mariadb (locked, 8.3, ubuntu-latest, 11.4)	53s
mysql (locked, 8.3, ubuntu-latest, 5.7)	58s
mysql (locked, 8.3, ubuntu-latest, 8.0)	59s
mysql (locked, 8.3, ubuntu-latest, 8.4)	1m 28s
mysql (locked, 8.3, ubuntu-latest, 9.0)	58s
Postgres (locked, 8.3, ubuntu-latest, 12.20)	1m 1s
Sqlite (locked, 8.3, ubuntu-latest)	14s
Postgres (locked, 8.3, ubuntu-latest, 13.16)	1m 25s
Postgres (locked, 8.3, ubuntu-latest, 14.13)	1m 23s
Postgres (locked, 8.3, ubuntu-latest, 15.8)	1m 26s
Postgres (locked, 8.3, ubuntu-latest, 16.4)	1m 27s
Postgres (locked, 8.3, ubuntu-latest, 17.0)	1m 26s
	18m 4s

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