refactor datastack-to-server cache

[ceesem]

Updated the datastack-to-server cache writing logic to only overwrite a datastack's value once a login was successful and to advertise the setup_token function if it was not successful.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
caveclient/frameworkclient.py	41.66%	7 Missing ⚠️

Files with missing lines	Coverage Δ
caveclient/datastack_lookup.py	40.67% <100.00%> (+1.02%)	⬆️
caveclient/frameworkclient.py	69.65% <41.66%> (-1.78%)	⬇️

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