pick some hot region optimization to release 2.1 (tikv#1551)

*: make hot region scheduler configurable (tikv#1412)

Signed-off-by: nolouch <nolouch@gmail.com>

* use high priority for the hot region related operators (tikv#1492)

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

* schedulers: let hot region balance not affect by balance-region-scheduler-limit (tikv#1522)

Signed-off-by: nolouch <nolouch@gmail.com>

statistic: add the statistic of flow (tikv#1548)

* store_statistic: add the statistic of flow

Signed-off-by: nolouch <nolouch@gmail.com>

server: set timeout for MoveLeader (tikv#1538)

Signed-off-by: disksing <i@disksing.com>

update CHANGLOG for 2.1.9 (tikv#1531)

server: fix cannot send error in heartbeat stream (tikv#1521)

Signed-off-by: nolouch <nolouch@gmail.com>

add CHANGELOG for 2.1.8 (tikv#1505)

Signed-off-by: nolouch <nolouch@gmail.com>

Bump to v2.1.7 (tikv#1484)

Signed-off-by: Neil Shen <overvenus@gmail.com>

Bump to v3.0.0-beta.1 (tikv#1480)

Signed-off-by: Neil Shen <overvenus@gmail.com>

update CHANGELOG (tikv#1446)

update CHANGLOG.md to release v2.1.3 (tikv#1419)

Signed-off-by: nolouch <nolouch@gmail.com>