Fix first query execution after a LOAD command

[rjuju]

If the extension is loaded with a plain LOAD command, the search_path will only be set during the next query execution. It means that the very first query executed after such a LOAD wouldn't see the global temporary tables.

[rjuju]

I found a corner case issue when trying some scenarios around issue #40.

Basically, doing:

LOAD 'pgtt';
SELECT * FROM some_global_temp_table;

would fail, as the search_path would be overloaded in the post_parse_analyze_hook of the SELECT query, which is never reached as it fails during the parse analysis, complaining about the global temp table not being found.