Better SHOW PROCESSLIST for replication

- When waiting for events, start time is now counted from start of wait
- Instead of having "Connect" as "Command" for all replication threads we
  now have:
  - Slave_IO for Slave thread reading relay log
  - Slave_SQL for slave executing SQL commands or distribution queries to
    Slave workers
  - Slave_worker for slave threads executin SQL commands in parallel replication

include/mysql_com.h

@@ -105,7 +105,10 @@ enum enum_server_command
   COM_STMT_RESET, COM_SET_OPTION, COM_STMT_FETCH, COM_DAEMON,
   /* don't forget to update const char *command_name[] in sql_parse.cc */
   COM_MDB_GAP_BEG,
-  COM_MDB_GAP_END=253,
+  COM_MDB_GAP_END=250,
+  COM_SLAVE_WORKER,
+  COM_SLAVE_IO,
+  COM_SLAVE_SQL,
   COM_MULTI,
   /* Must be last */
   COM_END