engine: new module for alert metrics

[dctalbot]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
This PR introduces a new engine module for creating and populating a new alert_metrics table. It prioritizes newly-closed alerts, but will eventually process all remaining closed alerts.

Which issue(s) this PR fixes:
Fixes #2121

How to review:

• make regendb
• See the theory of operation described in engine/metricsmanager/update.go
• Verify that the code implements the algorithm described
• make start
• Verify the new engine_processing_versions.state column is populated
• Verify the new alert_metrics table is populated

Additional Info:

• Every table must have a unique id column in order to be compatible with the switchover tool.
• A test for this was written and verified, but it is not included in this PR. We decided to wait until to graphql layer is implemented to avoid mixing the concerns of smoke tests.