This patch does refactoring necessary to support execution of DDL
commands on remote servers.
Basically it extends cross module api with ddl_command_start,
ddl_command_end and sql_drop functions.
Variable hypertables_list added to ProcessUtilityArg. It is used
to keep a list of found hypertables during Utility/DDL statement
parsing. This information and information gathered from other
hook functions will be used to distinct distributed hypertables
and forward DDL commands to any remote servers associated with
them.
