Commit b3f300b9 introduced the novel idea tracking oldest xmin
among all distributed snapshots on QEs.  However, the idea is not
applicable to QD because all distributed transactions can be found in
ProcArray on QD.  Local oldest xmin is therefore the oldest xmin among
all distributed snapshots on QD.  This patch fixes the maintenance of
oldest xmin on QD by avoiding DistributedLog_AdvanceOldestXmin() and all
the heavy-lifting that it performs.  Calling this on QD was also hitting
the "local snapshot's xmin is older than recorded distributed
oldestxmin" error occasionally in CI.