Miscellaneous Changes on 3.6.1

[yongxin-xu]

1. Bugfix of spider select with the group by and max()/min() returns error result.
2. Bugfix of select + distinct + order by(shard_key) + limit
3. Bugfix of the dangling pointer of spider_get_time.
4. Bugfix of circling lists in the two-phase commit. ha_register. (We did not reproduce the bug, instead we raise a solution to avoid it)
5. Default spider_max_connections is set to 500, which is to enable the connection pool. (In previous versions, this value is 0, which would lead to the OOM if the number of concurrencies is large)