gsqlbackend: remove hardcoded replaceRRSet queries
gsqlbackend: use boost::format in list()

Preparational commit.
gsqlbackend: reformat feedRecord

Those boost::format() calls with everything in one line are a pain
to read.
g*sql: put common query parts in a variable
gsql: stop hardcoding list-subzone-query
Implement "disabled" records for GSQL backends