Vacuum now uses distributed lowest dxid to decide oldest transaction *globally*
running in cluster to make sure tuple is DEAD globally before removing the
same. HeapTupleSatisfiesVacuum() consults distributed snapshot by reverse
mapping localXid to distributed xid to check xminAllDistributedSnapshots and
verifies its not needed anymore globally. Note the check is conservative from
perpective if cannot check against distributed snapshot (like utility mode
vacuum) will try to keep the tuple than prematurely getting rid of it and
suffering the same problem.

This fixes the problem of not removing the tuple still needed. Earlier it
performed the check just based on local information (oldestXmin) on a segment,
and hence may cleanup a tuple visible to a distributed query yet to reach the
segment, which breaks snapshot isolation.

Fixes #801.