Add a cluster monitor postmaster process

Right now the process is responsible for computing the local RecentGlobalXmin
and send periodic updates to the GTM. The GTM then computes a cluster-wide
value of the RecentGlobalXmin and sends it back to all the reporting nodes
(coordinators as well as datanodes). This way GTM does not need to track all
open snapshots in the system, which previously required a transaction to remain
open, even for a read-only operation. While this patch itself may not show
major performance improvements, this will act as a foundation for other major
improvements for transaction handling.

If a node gets disconnected for a long time or stops sending updates to the
GTM, such a node is removed from computation of the RecentGlobalXmin. This is to
ensure that a failed node does not stop advancement of the RecentGlobalXmin
beyond a certain point. Such a node can safely rejoin the cluster as long as
its not using a snapshot with a stale view of the cluster i.e. a snapshot with
xmin less than the RecentGlobalXmin that the GTM is running with.

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