Add CDB_EstimateRowCount function

See #295

test/CDB_EstimateRowCountTest.sql

@@ -0,0 +1,10 @@
+SET client_min_messages TO error;
+\set VERBOSITY terse
+CREATE TABLE tmptab1(id INT);
+INSERT INTO tmptab1(id) VALUES (1), (2), (3);
+CREATE TABLE tmptab2(id INT, value NUMERIC);
+INSERT INTO tmptab2(id, value) VALUES (1, 10.0), (2, 20.0);
+SELECT CDB_EstimateRowCount('SELECT SUM(value) FROM tmptab1 INNER JOIN tmptab2 ON (tmptab1.id = tmptab2.id);') AS row_count;
+SELECT CDB_EstimateRowCount('UPDATE tmptab2 SET value = 30 WHERE id=2;') AS row_count;
+DROP TABLE tmptab2;
+DROP TABLE tmptab1;