Enable supports_multivalues_insert in SQLAlchemy

[csringhofer]

Impala has already supported this, we simply had to turn on this capability in the Impala dialect.

Testing:

• added test to insert + select via SQLAlchemy

[csringhofer]

There was a previous PR for this, but it had no tests: https://github.com/cloudera/impyla/pull/499

An issue was also opened about this: https://github.com/cloudera/impyla/issues/354

[wzhou-code]

The new patch looks good. Thanks