rdbms: escape backslashes in makeConnectionString for PostgreSQL
Prior to this commit, only single quotes were escaped in
makeConnectionString. However, backslashes should be as well, as per the
documentation of pg_connect: "Single quotes and backslashes within the
value must be escaped with a backslash, i.e., \' and \\."
(cherry picked from commit eaa922d6217c7d64e129a7977a316791aa5863e5)