`set_keyspace` might flaky (causing a test to be flaky)

[fruch]

test_can_register_udt_before_connecting was flaky for a long time

we've pinpoint it to be the usage of set_keyspace and in https://github.com/scylladb/python-driver/pull/299 we stop using set_keyspace

this is the test before the change, for reference:

    def test_can_register_udt_before_connecting(self):
        """
        Test the registration of UDTs before session creation
        """
        c = TestCluster()
        s = c.connect(wait_for_all_pools=True)
        s.execute("""
            CREATE KEYSPACE udt_test_register_before_connecting
            WITH replication = { 'class' : 'SimpleStrategy', 'replication_factor': '1' }
            """)
        s.set_keyspace("udt_test_register_before_connecting")
        s.execute("CREATE TYPE user (age int, name text)")
        s.execute("CREATE TABLE mytable (a int PRIMARY KEY, b frozen<user>)")

        s.execute("""
            CREATE KEYSPACE udt_test_register_before_connecting2
            WITH replication = { 'class' : 'SimpleStrategy', 'replication_factor': '1' }
            """)
        s.set_keyspace("udt_test_register_before_connecting2")
        s.execute("CREATE TYPE user (state text, is_cool boolean)")
        s.execute("CREATE TABLE mytable (a int PRIMARY KEY, b frozen<user>)")

        # now that types are defined, shutdown and re-create Cluster
        c.shutdown()
        c = TestCluster()
        User1 = namedtuple('user', ('age', 'name'))
        User2 = namedtuple('user', ('state', 'is_cool'))
        c.register_user_type("udt_test_register_before_connecting", "user", User1)
        c.register_user_type("udt_test_register_before_connecting2", "user", User2)

        s = c.connect(wait_for_all_pools=True)

        s.set_keyspace("udt_test_register_before_connecting")
        s.execute("INSERT INTO mytable (a, b) VALUES (%s, %s)", (0, User1(42, 'bob')))
        result = s.execute("SELECT b FROM mytable WHERE a=0")
        row = result[0]
        self.assertEqual(42, row.b.age)
        self.assertEqual('bob', row.b.name)
        self.assertTrue(type(row.b) is User1)

        # use the same UDT name in a different keyspace
        s.set_keyspace("udt_test_register_before_connecting2")
        s.execute("INSERT INTO mytable (a, b) VALUES (%s, %s)", (0, User2('Texas', True)))
        result = s.execute("SELECT b FROM mytable WHERE a=0")
        row = result[0]
        self.assertEqual('Texas', row.b.state)
        self.assertEqual(True, row.b.is_cool)
        self.assertTrue(type(row.b) is User2)
        s.execute("DROP KEYSPACE udt_test_register_before_connecting")
        s.execute("DROP KEYSPACE udt_test_register_before_connecting2")

the hypophysis I have, is that some connection (of some shard) might have been missed, and didn't got the USE command, we don't have a mechanism to validate all connect got it, and that the next query can't be executed before they did