Re: Make COPY format extendable: Extract COPY TO format implementations

One problem in the following chunk I can see is:

+           qualified_format = stringToQualifiedNameList(format, NULL);
+           DeconstructQualifiedName(qualified_format, &schema, &fmt);
+           if (!schema || strcmp(schema, "pg_catalog") == 0)
+           {
+               if (strcmp(fmt, "csv") == 0)
+                   opts_out->csv_mode = true;
+               else if (strcmp(fmt, "binary") == 0)
+                   opts_out->binary = true;
+           }

Non-qualified names depend on the search_path value so it's not
necessarily a built-in format. If the user specifies 'csv' with
seach_patch = 'myschema, pg_catalog', the COPY command unnecessarily
sets csv_mode true. I think we can instead check if the retrieved
handler function's OID matches the built-in formats' ones. Also, it's
weired to me that cstate has csv_mode and binary fields even though
the format should have already been known by the callback functions.