Add a developer-GUC 'gloabl_snapshot_source' to allow informed users to

override the way snapshots are computed.

The default value of the GUC is 'gtm' which means that snapshots are always
generated on the GTM so that we get a full and correct view of the currently
running transactions. But with this developer-GUC we now allow users to
override that and work with a coordinator generated snapshots. This can be
especially useful for read-only queries which now don't need to talk to the
GTM. If the snapshots can also be taken locally on a coordinator, this will
even further reduce the round-trips to the GTM. Of course, this can lead to
consistency issues because a coordinator may not be aware of all the
transactions currently running on the XL cluster, especially in a
multi-coordinator setup where different coordinators could be running different
transactions without knowing about each other's activity. But even in a single
coordinator setup, some transactions may start on a datanode and coordinator
may not know about them or may only know quite late.

Its advised that this feature must be used with caution and after due
consideration of the effects

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