Commit 576690f2 added tracking of which segments have been enlisted in
a distributed transaction. However, it registered every query dispatched
with CdbDispatch*() in the distributed transaction, even if the query was
dispatched without the DF_NEED_TWO_PHASE flag to the segments. Without
DF_NEED_TWO_PHASE, the QE will believe it's not part of a distributed
transaction, and will throw an error when the QD tries to prepare it for
commit:

ERROR:  Distributed transaction 1542107193-0000000232 not found (cdbtm.c:3031)

This can occur if a command updates a partially distributed table (a table
with gp_distribution_policy.numsegments smaller than the cluster size),
and uses one of the backend functions, like pg_relation_size(), that
dispatches an internal query to all segments.

Fix the confusion, by only registering commands that are dispatched with
the DF_NEED_TWO_PHASE flag in the distributed transaction.
Reviewed-by: NPengzhou Tang <ptang@pivotal.io>