Fix memory leak when query cache enabled in streaming replication mode + extended...

Fix memory leak when query cache enabled in streaming replication mode + extended query case.

If BEGIN is executed in extended protocol, allocated temp buffer for
the command never discarded.  This is because in streaming replication
mode + extended query case, when handle_query_cache gets called at the
command complete timing, the reference counter to query context is not
0, which in turn prevents from calling
pool_discard_temp_query_cache().

In reality, we don't need to care about the query context here, so
let's proceed in streaming replication mode + extended query case.

Note that the memory leak case can be easily reproduced by "pgbench -M
extended" command (standard TPC-B).