Fix resource leak and improved wait_for_ready

[RunDevelopment]

I noticed that reloading the frontend would sometimes cause the host server to not respond anymore even though the worker was fine. The cause of this issue was a resource leak with our _session client session. Turns out, everything an aiohttp.ClientSession returns is a resource that must be disposed of. We were not doing that in a few methods, and aiohttp doesn't do auto-clean up apparently, so after a few leaks the entire backend host server would become unresponsive as _session would always timeout.

Leaks were hard to reproduce btw. I had to do 5 leaky requests to the worker's /nodes in parallel for to consistently trigger the bug.

The fix was to properly manage resource everywhere. I also improved wait_for_ready to not spam the worker with requests by (1) remembering whether we successfully connected to it before and (2) making processing concurrent calls to wait_for_ready sequentially which benefits from (1).

[joeyballentine]

Thanks, this might fix the weird stuff I was seeing the past few days