Decouples the postgresql sql generation from the query execution.
This allows the logic for building sql to be unit tested without
access to a database.
This work was motivated when a client was not receiving events as
expected. In debugging I found that if a tag's value was an empty array,
then no query would be executed - and to my surprised no error is
raised either. I wanted to get a better sense of the current constraints on
when queries are and are not executed, but I had a hard time keeping the
code in my head. This led me to extracting the sql generation into its
own function and writing the unit tests that document its current
behavior. This refactor makes no changes to the current logic. I have added
some REVIEW comments in the test cases where I thought some error handling
could be introduced but I wanted to first see if you were receptive to this
refactor before proposing any functional changes.
