Resolve independent cluster test failures

[charlesbluca]

Fixes breakage we've been seeing in independent cluster testing due to:

• mismatched pandas versions (resolved with Unpin dependencies dask/dask-docker#209)
• the bump to distributed 2022.3.0

In particular, one big change we've made is that we now use a single session-wide client to perform all computations or random client operations - backed either by a local cluster or an independent cluster for our independent cluster testing. I've updated the handful of tests that manage client lifecycle stuff to avoid this causing larger failures, but it would be nice at some point to explore better scope options for the client fixture.

cc @ayushdg

[ayushdg]

Thanks for the quick fix! This should be good in the short term for unblocking CI

[codecov-commenter]

Codecov Report

Merging #437 (7c75468) into main (157b3f4) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #437      +/-   ##
==========================================
- Coverage   89.02%   88.92%   -0.10%     
==========================================
  Files          69       69              
  Lines        3326     3324       -2     
  Branches      653      647       -6     
==========================================
- Hits         2961     2956       -5     
  Misses        296      296              
- Partials       69       72       +3     

Impacted Files	Coverage Δ
dask_sql/server/app.py	96.96% <0.00%> (-3.04%)	⬇️
dask_sql/_version.py	32.27% <0.00%> (-0.29%)	⬇️
dask_sql/physical/rel/custom/predict.py	95.34% <0.00%> (-0.11%)	⬇️
dask_sql/utils.py	100.00% <0.00%> (ø)

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