Not compatible with MySQL 8.0

[px3303]

MySQL 8.0 has introduced cache to improve the efficiency of information_schema queries, so it may lead to a consequence that users could not get the latest and accurate statistics when querying information_schema.

If using MySQL 8.0 as storage layer, trillian codes, without setting the session variable information_schema_stats_expiry (the default is 86400 seconds), could not pass the TestMySQLRateLimiting and TestQuotaManager_GetTokens_InformationSchema.

--- FAIL: TestMySQLRateLimiting (1.13s)
    quota_test.go:100: Test MySQL available at "root@tcp(127.0.0.1)/"
    quota_test.go:124: timed out before rate limiting kicked in
FAIL
FAIL    github.com/google/trillian/integration/quota    8.509s

--- FAIL: TestQuotaManager_GetTokens_InformationSchema (1.44s)
    mysql_quota_test.go:118: Test MySQL available at "root@tcp(127.0.0.1)/"
    --- FAIL: TestQuotaManager_GetTokens_InformationSchema/useSelectCount_=_false (1.34s)
        mysql_quota_test.go:170: timed out
FAIL
FAIL    github.com/google/trillian/quota/mysqlqm        4.407s

The countFromInformationSchema function in the codes depends on information_schema to return the latest statistics,

	countFromInformationSchemaQuery = `
		SELECT table_rows
		FROM information_schema.tables
		WHERE table_schema = schema()
			AND table_name = ?
			AND table_type = ?`

func countFromInformationSchema(ctx context.Context, db *sql.DB) (int, error) {
	// information_schema.tables doesn't have an explicit PK, so let's play it safe and ensure
	// the cursor returns a single row.
	rows, err := db.QueryContext(ctx, countFromInformationSchemaQuery, "Unsequenced", "BASE TABLE")
	if err != nil {
		return 0, err
	}
	defer rows.Close()
	if !rows.Next() {
		return 0, errors.New("cursor has no rows after information_schema query")
	}
	var count int
	if err := rows.Scan(&count); err != nil {
		return 0, err
	}
	if rows.Next() {
		return 0, errors.New("too many rows returned from information_schema query")
	}
	return count, nil
}

so anything that relies on this function may fail to work as normal.