Refactor SQL-based event log storages to use SQLAlchemy
Summary: This resolves https://github.com/dagster-io/dagster/issues/1798 as well as performance issues stemming from calls to the default implementation of get_stats_for_run. Replaces D1457, stacked on D1470, D1472, D1473, D1474.
Test Plan: Unit
Reviewers: alangenfeld, prha, nate, schrockn, themissinghlink
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D1471