memory leaking
* Delete neu_dvalue_array_new and neu_dvalue_vec_new
* Delete set/get function for vec and array
* Add free sub data value in array and vector
* Add subscribe/unsubscribe node in command and request interface.
* Add neuron request structure for subscribe/unsubscribe node.
* Add subscribe node before enter message loop.
* Add unsubscribe node after exit message loop.
* Add compare function for primitive value.
* Add vector_find_index/vector_find_item/vector_has_elem util function
for vector.
* Forward subscribe/unsubsribe message in manager, and make sure to
forward the subscribe/unsubsribe message only once.
* Add neu_int_val_t and neu_string_val_t structures and functions
* Add functions for set/get int-val and string-val in data value
* Add functions for supoort serial/deserial int-val and string-val
add NEU_REQRESP_GET_SELF_NODE_ID cmd
add get_datatags_table func
define neu_addt_str_t
fix compile warning
add magic number and check function in neu_plugin_common_t
1. Variable array serialization and deserialization support
2. Variable basic data type setting
3. Variable export function unit test
4. Integrate MQTT and Variable
5. Delete the work queue in the MQTT driver