Build a generic database upgrade mechanism

[bgrieder]

A generic database upgrade mechanism on startup should be built.

This involves:

• Creating a context table with a single record holding the last version of the software run (when the table is not present, assume the version run is the one before the version implementing the change)
• On server startup:
  • the server checks if the software version is greater than the last version run
  • if no, it simply starts
  • if yes,
    • it looks for all upgrades to apply in order from the last version run to this version (this is done using a static function hardcoding the upgrade functions to call for each version)
    • if there is any to run, it sets an upgrading flag on the db state field in the context table
    • it runs all the upgrades in order
    • it sets the flag from upgrading to ready
• On every call to the DB, a check must be performed on the db state field to check if the DB is upgrading. If yes, calls should fail. Performance can be maintained using a secondary write-through cache of the context table

Upgrades should be programmed to resist being interrupted in the middle and resumed from start.