client: revise pdclient dc dispatcher (#3413)
client: revise pdclient dc dispatcher (#3413)

* revise client

Signed-off-by: Song Gao <disxiaofei@163.com>

* fix dispatch

Signed-off-by: Song Gao <disxiaofei@163.com>

Co-authored-by: Ti Chi Robot <72214b26dd6f24c8c411751e1f8e05856122286e@users.noreply.github.com>
pingcap/pd,pingcap/pd