Extract important queries to postgres functions

[lorenzo]

The motivating use case for extracting these functions is that it gives more control to the developer using this package on how to deploy this to their infrastructure.

For instance, some people may be interested in a different table structure for payloads, like having it partitioned. With this change, developers are free to change the implementation of the postgres functions as they see fit as long as they implement an equivalent logic and return the same types.