[enhancement] Result database performance improvements

[vkarak]

This PR adds two significant performance improvements for DB queries involving session information.

1. It adds an index on the session's start time in the sessions table. This significantly improves the session queries as well as test case queries that use session filtering criteria.
2. The --describe-stored-sessions option performance is considerably improved as the unnecessary decoding and encoding of the results is removed. Since the result is already going to be returned as JSON, there is no need to pay the cost of decoding and re-encoding it, so it is returned raw from the database.

Another minor improvement was also done to further optimize the JSON decoding inside the backend.

More specifically in a 15GB database and the query spanning roughly 3K sessions was ~30x faster using the index:

reframe --list-stored-testcases='now-7d:now?ci_pipeline_id=="123456789"'/mean:/+presult

where ci_pipeline_id is custom session property stored in the DB (see --session-extras).

The --describe-stored-session with a similar span of records was 3x faster using the improvements from this PR.

Finally, the index addition is seamless. The first time a query to the DB will be made with the new version the necessary DB indexes will be created if they do not exist. Note this very first query make take some time to execute depending on the size of the DB.

[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.21%. Comparing base (f9c3c56) to head (590db46).
⚠️ Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
reframe/frontend/reporting/utility.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3546   +/-   ##
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  Coverage    91.21%   91.21%           
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  Files           62       62           
  Lines        13451    13462   +11     
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+ Hits         12269    12280   +11     
  Misses        1182     1182           

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