For FQSed query, run EXPLAIN on the remote node and print the result
For FQSed query, run EXPLAIN on the remote node and print the result

We recently added fast-query-shipping facility to Postgres-XL. But it wouldn't
print the query plan used on the datanodes, which makes it extrememly difficult
to debug bad query plan with EXPLAIN. We now send an EXPLAIN query to one of
the datanodes and print the result. This assumes that all datanodes would plan
queries similarly, assuming a good data distribution.

We also modified the EXPLAIN output for FQSed queries to match is as closely as
possible with the Remote Subplan queries.

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl