Beaker workspace performance

[epwalsh]

Fixes #326 (hopefully)

Changes proposed in this pull request:

• Upgrades beaker-py to minimum v1.6.2 in order to get this essential bug fix: allenai/beaker-py@d3f18dc.
• Bumps the number of thread pool workers in BeakerWorkspace.
• Uses a single requests.Session() object within the Beaker client to address the "too many open files" issue ("Too many open files" while using BeakerExecutor #326) and improve performance.

⚠️ We're assuming requests.Session() is thread safe for this use-case, even though the maintainers of requests don't provide any guarantees of thread safety. That said, following the discussion in psf/requests#2766, it seems the main reason for concern is the lack of thread safety in urllib3.PoolManager (psf/requests#2766 (comment)). The thread-safety issue with the pool manager seems to boil down to this: urllib3/urllib3#1252 (comment). That is, when the number of hosts is greater than the number of pools. But the default number of pools is 10 (https://github.com/psf/requests/blob/main/requests/adapters.py#L66), and in our case there are only two different hosts that we connect to: beaker.org and the Beaker file heap server. So we should be okay 🤷‍♂️

If we do run into issues in the future, we should handle this in beaker-py but using one session per thread. We could implement this using thread-local storage similar to this: https://github.com/customerio/customerio-python/pull/77/files

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