sa: centralize lists of columns for object types

[jsha]

In #4880, we fixed an issue where the list of columns for a certificateStatus was different in ocsp-updater than it was in the SA. We should make the SA the authoritative source of column lists for objects, and make sure that any place we explicilty enumerate columns (rather than leaving it to Gorp), we get the list from the SA package.

[jsha]

Work still to be done: Find places that use ad-hoc structs as receivers of database queries, replace those with methods under sa package.