New UDF citus_get_cluster_clock() that returns a monotonically increasing logical clock

[tejeswarm]

This implements a new UDF citus_get_cluster_clock() that returns a monotonically increasing logical clock. Clock guarantees to never go back in value after restarts and makes best attempt to keep the value close to unix epoch time in milliseconds.

Introduces a new GUC "citus.enable_cluster_clock", when true, every distributed transaction is stamped with logical causal clock and persisted in a catalog pg_dist_commit_transaction.

DESCRIPTION: This implements a new UDF citus_get_cluster_clock() that returns a monotonically increasing logical clock