Fix a long standing bug in vacuum/analyze of temp tables

Fix a long standing bug in vacuum/analyze of temp tables

The system may and very likely choose different namespace for temporary tables
on different nodes. So it was erroneous to explicitly add the coordinator side
nampspace to the queries constructed for fetching stats from the remote nodes.
A regression test was non-deterministically failing for this reason for long,
but only now we could fully understand the problem and fix it. We now use
pg_my_temp_schema() to derive the current temporary schema used by the remote
node instead of hardcoding that in the query using coordinator side
information.