Explicitly track if a subplan has been executed and a valid value is computed

Explicitly track if a subplan has been executed and a valid value is computed
or not.

This works around a problem noted in issue #102, but not a full solution to the
problem. It seems there are places where InitPlan is attached to a node which
will be executed on the remote node. Even so coordinator tries to find push a
PARAM_EXEC parameter value for a remote subplan and fails because the value is
not and will never be computed on the coordinator. We for now send NULL value
for such cases to avoid a server crash