Add a non-disruptive reload option

[Joe Thompson]

Since the product-api service connects to a database, the connection string may change if the database itself or the credentials to access it change -- this is commonly the case with Vault dynamic database credentials. Currently it is not possible to get the app to reload its config except by completely restarting it; a reload/reconnect on receipt of a HUP or USR1 process signal or triggered by a call to an API endpoint would make the app much smoother to use with something like Vault Agent.