BigQuery: Change the default value of Cursor instances' arraysize attribute to None

[plamut]

Closes #9185.

This PR adds a note on arraysize parameter, and its impact on the fetchall() method performance.

To discuss

• Following a comment on the issue, should we reanimate the fetchmany()'s size parameter, too?
  No (for the time being), as that would require implementing a custom pagination logic in the client (comment).

How to test

Run the code sample from the issue description, and verify that by setting cursor.arraysize to appropriate value avoids the reported performance issue, which what the updated docs point out.
Run the code sample from the issue description, and verify that the new default value of cursor.arraysize avoids the reported performance issue. Or setting that value manually to something appropriate.

[tswast]

completely forgot about the arraysize parameter!

Rather than add this note, I think it may make more sense to diverge from the DB-API spec (which dictates a default arraysize=1).

google-cloud-python/bigquery/google/cloud/bigquery/dbapi/cursor.py

Lines 63 to 65 in cf44700

	# Per PEP 249: The arraysize attribute defaults to 1, meaning to fetch
	# a single row at a time.
	self.arraysize = 1

If we instead default to None, the BigQuery API backend can choose a more appropriate page size automatically.

[plamut]

If we instead default to None, the BigQuery API backend can choose a more appropriate page size automatically.

I see, that actually makes sense. It's safer than just having it in the docs, as some portion of the users will probably still overlook it.