Show memory usage in show processlist

[BohuTANG]

Summary

After #2299 and #2356 track a session memory usage, it would be better to add the memory usage to the show processlist:

mysql> SHOW PROCESSLIST;
+--------------------------------------+-----------------+-------+----------+------------------+-------------+
| id                                   | host            | state | database | extra_info       | memory_usage|
+--------------------------------------+-----------------+-------+----------+------------------+--------------+
| 1e6e5ed4-5441-43da-9ed6-eb6ba9baeb64 | 127.0.0.1:60080 | Query | default  | show processlist | 123456 |
| 3d283add-4f60-416d-b9ca-662120614093 | 127.0.0.1:57018 | Query | default  | NULL             | 123456 |
+--------------------------------------+-----------------+-------+----------+------------------+-------------+

[zhang2014]

It is already implemented in main branch.