BatchQuery serializes batch to string instead of using BatchStatement

[Lorak-mmk]

In the cqlengine, there is BatchQuery class -

python-driver/cassandra/cqlengine/query.py

Line 136 in cd1f5ed

class BatchQuery(object):

- which can be used as a context manager and with other cqlengine constructs.
For some reason, in it's execute method it serializes whole batch to the string - instead of using BatchStatement

python-driver/cassandra/query.py

Line 710 in cd1f5ed

class BatchStatement(Statement):

which is a protocol-level batch.
This is 1. bad in terms of performance 2. causes other issues - e.g. here

python-driver/cassandra/cluster.py

Line 5366 in cd1f5ed

is_batch_statement = isinstance(self.response_future.query, BatchStatement)

the driver checks if query is a batch by checking if it's a subclass of BatchStatement - and queries created with BatchQuery obviously won't.

Unless there is some fundamental reason for this behavior, we should fix it.