Fix MySQL connection drop

[panakouris]

This fixes #266.
Set pool_pre_ping to True when creating the sqlalchemy engine as discussed in #266

[codecov]

Codecov Report

Merging #267 (bd13280) into main (2e8bcdb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #267   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files          49       49           
  Lines        2211     2211           
  Branches      310      310           
=======================================
  Hits         2184     2184           
  Misses         17       17           
  Partials       10       10           

Impacted Files	Coverage Δ
terracotta/drivers/relational_meta_store.py	98.93% <ø> (ø)

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