*: fix race in hot region config (#4167)
*: fix race in hot region config (#4167)

* fix race in hot region config

Signed-off-by: Ryan Leung <98ca95cd24c810cea97a0e53f67d648652b7b2cf@gmail.com>

* address the comment

Signed-off-by: Ryan Leung <98ca95cd24c810cea97a0e53f67d648652b7b2cf@gmail.com>
pingcap/pd,pingcap/pd