control-connection fallback: preserve session keyspace

[dkropachev]

Problem

A session created with cluster.connect("ks") stores its keyspace:

python-driver/cassandra/cluster.py

Lines 1744 to 1749 in b666e5c

	def connect(self, keyspace=None, wait_for_all_pools=False):
	"""
	Creates and returns a new :class:`~.Session` object.
	If `keyspace` is specified, that keyspace will be the default keyspace for
	operations on the ``Session``.

python-driver/cassandra/cluster.py

Lines 2619 to 2622 in b666e5c

	def __init__(self, cluster, hosts, keyspace=None):
	self.cluster = cluster
	self.hosts = hosts
	self.keyspace = keyspace

When no node pool is available, fallback sends directly through the shared control connection without applying that keyspace:

python-driver/cassandra/cluster.py

Lines 5065 to 5096 in b666e5c

	def _query_control_connection(self, message=None, cb=None, connection=None, host=None):
	self._control_connection_query_attempted = True
	if message is None:
	message = self.message
	if connection is None:
	control_connection = self.session.cluster.control_connection
	connection = control_connection._connection if control_connection else None
	if not connection:
	self._errors['control connection'] = ConnectionException("Control connection is not connected")
	return None
	if host is None:
	host = self.session.cluster.get_control_connection_host() or connection.endpoint
	self._current_host = host
	request_id = None
	request_sent = False
	try:
	request_id = self._borrow_control_connection(connection)
	self._connection = connection
	result_meta = self._bound_result_metadata
	if cb is None:
	cb = partial(self._set_result, host, connection, None)
	cb = partial(self._handle_control_connection_response, connection, cb)
	log.debug("No usable node pools; falling back to control connection for host %s", host)
	self.request_encoded_size = connection.send_msg(message, request_id, cb=cb,
	encoder=self._protocol_handler.encode_message,
	decoder=self._protocol_handler.decode_message,
	result_metadata=result_meta)

Unqualified queries and prepares can fail or execute against the keyspace selected by another session.

Expected behavior

Preserve each session's keyspace when dispatching through the control connection, without leaking keyspace state between sessions. Add coverage for two sessions using different keyspaces.