Add a citus_use_snapshot function for basic distributed snapshot isolation

[marcocitus]

DESCRIPTION: Adds a citus_use_snapshot UDF for basic distributed snapshot isolation

This PR implements a basic form of distributed snapshot isolation in a way that's similar to citus_create_restore_point. The citus_use_snapshot UDF:

1. Locks pg_dist_transaction on all nodes
2. Calls pg_export_snapshot on all nodes
3. Unlocks pg_dist_transaction
4. Uses the exported snapshots for all remote transaction blocks

By taking the pg_dist_transaction lock, we ensure that all ongoing 2PCs have finished when we do pg_export_snapshot, so the queries in the transaction block will not see any partially committed transactions. The lock is released immediately so it only briefly interrupts multi-shard writes in their final stages of commit.

The citus_use_snapshot UDF can only be used in REPEATABLE READ (or SERIALIZABLE) mode at the start of a transaction block. Currently, multi-shard writes are not supported due to a postgres limitation: cannot PREPARE a transaction that has exported snapshots.

Usage:

DROP TABLE IF EXISTS test;
CREATE TABLE test (key int primary key, value int);
INSERT INTO test SELECT s, 0 FROM generate_series(1,100) s;
SELECT create_distributed_table('test','key');

BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SELECT citus_use_snapshot();
SELECT sum(value) FROM test; 
END;

generate concurrent multi-shard updates that should leave sum at 0:

$ cat test.sql 
\set aid random(1, 100)
\set bid random(1, 100)
BEGIN;
UPDATE test SET value = value + 1 WHERE key = :aid;
UPDATE test SET value = value - 1 WHERE key = :bid;
END;
$ pgbench -c 20 -j 4 -f test.sql -P 10 -T 60 -n

(will have some regular deadlocks)