[bug] reflow v2 close ownership

[blast-hardcheese]

Why

Addressing some dangling work to hopefully advance the v2 client stability.

The main issue is that Session.close() had this weird circular dependency with ClientTransport.close() which increased the chance of error.

It should have been ClientTransport.close() -> Session.close() with no backwards arrows.

What changed

• Move Session.close() responsibility into ClientTransport
• Switch print(...) to logging.debug to make it easier to --log-level=debug in pytest and see all the logs together.
• get_current_time() incorrectly used asyncio.Task's internal clock, instead of the wall clock, extending timeouts further than they should have been.
• Caught a bug from a prior refactor where the deadline calculation was done inside the retry loop instead of before entering the loop

Test plan

CI, manual testing with staff/explorers after releasing a new version