Address use of UUIDs

[zakpatterson]

All modules at this stage use Postgres UUIDs as primary keys. Not all users will want this, as it adds a small amount of storage overhead, and might impact the way that postgres builds indexes.

So if there could be a way to allow the user to configure these and use sequences or whatever else for modules that already exist, that would be good. Or, we should just plant our flag and document the reasoning for using UUIDs for all the primary keys.