In this commit, all the sql, non-postgres-specific, code is moved out of
the postgres package and into a new sqlbase package. This will make it
more easily reusable for future sql integrations.
In this commit, changes are made to the `kvdb/postgres` package so that
all all the non-postgres-specific code is generalised to be applicable
for all sql code. A follow up commit will move all the general sql code
into its own package.
Due to the update of the kvdb package to go1.18, the `// +build`
directives are no longer required. They are removed in this commit in
order to simplify upcoming commits. Along the way, a few typos are fixed
and an unused struct is removed.
