Support Pipeline/Batch mode

[dvarrazzo]

In this mode, several cursor may send commands to the same connection and then sit in a queue to receive the result.

https://www.postgresql.org/docs/14/libpq-pipeline-mode.html

[dlax]

@dvarrazzo, as I'm progressing on this, I'm wondering where we should put tests for this feature. Would they live alongside other tests in test_connection.py, test_cursor.py, test_transaction.py, test_server_cursor.py (and async siblings)? Or should we just add a test_pipeline.py file?

[dvarrazzo]

Certain major features have their own test file, because it made more sense working focused on that (e.g. test_prepared.py). If you find more convenient to have test_pipeline.py and test_pipeline_async.py. I think it makes sense so.

[dvarrazzo]

Finally closed in #175.

Thank you very much, @dlax. This was an epic work!