DM-44050: Configure Postgres connection pool parameters

[dhirving]

Adjusted SQLAlchemy's connection parameters for postgres:

• Enabled pool_pre_ping to prevent exceptions from being thrown due to stale DB connections.
• Increased the pool size from 1 to 10, to prevent connections from being re-created excessively in multi-threaded services.

The pool_size increase is less risky than it at first appears... only a handful of services are constructing Butlers in a way that causes more than 1 connection to ever be created in the pool:

• At SLAC, it's just the Butler server which has only 1 instance and is rarely used currently.
• At the summit, it is only exposurelog. This service is pretty low concurrency. They are going to do a Butler update soon to get pool_pre_ping, so I will let them know to watch out for other side effects in their testing.
• If changing this breaks IDF we want to know that now, because it means we likely have serious scalability problems.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.04%. Comparing base (29714cd) to head (613a1ed).

Additional details and impacted files

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  Coverage   89.04%   89.04%           
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  Files         347      347           
  Lines       44473    44473           
  Branches     9146     9146           
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  Hits        39601    39601           
  Misses       3550     3550           
  Partials     1322     1322           

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[andy-slac]

Looks great! I'm a bit concerned with a hard-coded pool_size value, it could happen that for some services we may want to play with this parameter. It would be nice to have an option for updating that on client side, but our current model does not support it.