Add REFRESH FUNCTIONS <db> statement

Currently, Impala relies on INVALIDATE METADATA to load externally added UDFs from HMS. However, INVALIDATE METADATA affects all the databases/tables of an Impala cluster. Instead, we should add a REFRESH FUNCTIONS <db> statement to reload the functions of a database from HMS. This statement should return the list of db functions to the issuing Impalad in order to update its local catalog cache.