Fix "pg_bench -C -M prepared".
Fix "pg_bench -C -M prepared".

This didn't work because when we dropped and re-established a database
connection, we did not bother to reset session-specific state such as
the statements-are-prepared flags.

The st->prepared[] array certainly needs to be flushed, and I cleared a
couple of other fields as well that couldn't possibly retain meaningful
state for a new connection.

In passing, fix some bogus comments and strange field order choices.

Per report from Robins Tharakan.

50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,jmcatamney/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,ashwinstar/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb