fix: avoid blocking in download thread when using BQ Storage API

[kien-truong]

This prevents a deadlock between the main thead and download threads when the threadpool is shutdown prematurely.

Fixes#2032 🦕

[kien-truong]

Just dropping by to see if there's any concerns about this PR.

[chalmerlowe]

I took a quick look.
Gonna run our CI/CD checks on this.
My first thought is that because we are adding a couple code paths coverage is likely to fail.
Also curious why the pragma for queue.full: we recently got bitten by having a code branch that was not tested and I am hesitant about adding more such paths to the code base without understanding for myself why it is necessary/desireable.

[kien-truong]

The queue.Full code path is hard to test because it's non-deterministic: the queue can be full or not depending on the timing of consumers and producers.

So, I just put a pragma there to ignore the coverage, similar to the existing code a few lines below.

python-bigquery/google/cloud/bigquery/_pandas_helpers.py

Line 968 in aaec3ce

exceptqueue.Empty: # pragma: NO COVER

[tswast]

Looks like cover is passing.

[tswast]

After sleeping on this, I do think it's important to try and cover the currently uncovered branch. I'll see what I can do in a follow-up PR today.

Edit: mailed #2127