Pluggable WHC and RHC

[cwyark]

Connect to these issues:

CycloneDDS

• About dynamic memory (de)allocation #99

rmw_cyclonedds

• On allocations in the data path ros2/rmw_cyclonedds#79

ROS2 doesn't have any concept of instance or topic key, there shouldn't such complex DDS WHC/RHC implementation for ROS2 case. Also making so much memory (de)allocation might be harmful for the message transmit / receive path from ROS2 (rclcpp or rclpy) to CycloneDDS. So connect to #99 and ros2/rmw_cyclonedds#79, making pluggable whc/rhc in CycloneDDS and a custom whc/rhc in rmw_cyclonedds might be a good point to start the realtime improvement.

Since rhc has already become pluggable, whc could have the same change.
Here's my approach,

1. Move all whc/rhc implementation to particular folder, let this folder maintain all whc/rhc impl methods, e.g. default impl, or static memory allocation impl.
2. Whc part, remove q_whc.c and q_whc.h and move them to whc default impl.
3. Rename all whc relevant types to dds_whc if they are defined in dds_whc.h
4. Replace the original dds_create_writer with dds_create_writer_int , and add dds_create_writer_whc and dds_create_writer, like dds_create_reader did before.