[YSQL] Prepared statements not invalidated after schema change

[srhickma]

Jira Link: DB-1355
Summary: In some cases, if a statement is prepared on one node, and a table used in the statement is altered from another node, the prepared statement will not be invalidated. This issue can happen even if enough time has passed for the t-server to heartbeat to the master and receive the updated catalog version number.

Test Case:

 @Test public void testConsistentPreparedStatements() throws Exception {
    try (Statement statement1 = createConnection(0).createStatement();
         Statement statement2 = createConnection(1).createStatement()) {
      // Create table from connection 1.
      statement1.execute("CREATE TABLE test_table(id int, PRIMARY KEY (id))");

      // Wait for heartbeat.
      Thread.sleep(1000);

      // Force a cache refresh on connection 2.
      statement2.execute("SELECT * FROM test_table");

      // Prepare from connection 2.
      statement2.execute("PREPARE plan (int) AS SELECT * FROM test_table where id=$1");

      // Alter table from connection 1.
      statement1.execute("ALTER TABLE test_table ADD COLUMN b int");
      statement1.execute("INSERT INTO test_table(id, b) VALUES (1, 2), (2, 3)");

      // Wait for heartbeat.
      Thread.sleep(1000);

      // Only one column is returned.
      assertQuery(statement2, "EXECUTE plan(1)", new Row(1));
    }
  }

Expected Result: The final query in the test should fail with error "ERROR: cached plan must not change result type". This is the error produced if all queries are executed using the same statement (and connection).

Actual Result: The test passes.

Note: This problem also occurs if the statement is prepared after the alter table statement, but before the next t-server heartbeat.