[Small Ticket] Azure SQL Database: define a "load.schema" and move there "country" and "source"

[jmargutt]

Right now in load._read_db we have two fields, country and source, which are hardcoded but should not be. Also, one might want to define other fields (e.g. emergency) to be used in the database.

Therefore, we should define a "schema" in the Load class which specifies which fields have to be there in the messages to read/write from/to the database, and, if empty, fill them accordingly.