UniStore: Use epoch with microsecond resolution as RV (#92638)

* Use epoch with microsecond resolution as RV

* fix backend tests

* Add solution for when the clock goes back

* Add solution for when the clock goes back

* generate mocks

* go lint

* remove comment

* Use Greatest instead of max in msyql and postgres

* update tests

* Update pkg/storage/unified/sql/sqltemplate/dialect_sqlite.go

Co-authored-by: Diego Augusto Molina <diegoaugustomolina@gmail.com>

* cast to bigint

* add additional round trip

* increment the RV using 2 sql round trips instead of 3

* cleanup comments

* cast unix timestamp to integer

* fix postgres query

* remove old increment test data

* remove greatest

* cast unix_timestamp to signed

* Use statement_timestamp instead of clock_timestamp

---------

Co-authored-by: Diego Augusto Molina <diegoaugustomolina@gmail.com>

pkg/storage/unified/sql/sqltemplate/dialect_mysql.go

@@ -33,3 +33,7 @@ func (backtickIdent) Ident(s string) (string, error) {
 		return s
 	})
 }
+
+func (mysql) CurrentEpoch() string {
+	return "CAST(FLOOR(UNIX_TIMESTAMP(NOW(6)) * 1000000) AS SIGNED)"
+}