Connection pooling and automate Flway migration

[steven-sheehy]

• Add HikariCP database connection pooling
• Going forward, please do not keep Connection objects open after use or store them in class variables. Use them locally in try-with-resource so they close automatically after every related batch of statements. Connection pooling doesn't actually close under the covers, just puts it back in the pool.
• Moves Flyway based DB migrations to code. First process up will lock table and run migration.
• Adds apiUsername, apiPassword and dbName to config.json to support Flyway placeholders in SQL migration scripts.
• Adds restart: unless-stopped to containers so they restart after a crash
• Increases PostgreSQL stop grace period from 10s to 2m so it has more time to shutdown gracefully

Fixes #32

[gregscullard]

Updated config.json.sample to add the parameters that ConfigLoader looks for if environment variables aren't set.