merge master to 2.0 to release rc5 (tikv#1023)

merge master to 2.0 to release rc5 (tikv#1023)

merge master to 2.0 to release rc5 (tikv#1023)

ThreadSafeCache Get method lock bug (tikv#1008)

schedule: add region merge checker (tikv#839)

* schedule: add region merge checker

* fix get adjacent region and add test

* fix check key range hole and add test

* update kvproto

* fit for kvproto

* tiny adjust

* check key range hole in getAdjacentRegions

* check operators before add

* IsFinish check if key range expands

* change influence

* add merge operator test

* make merge-region-size unconfigurable temporarily

* check timeout operator in background

* tiny clean

* add comment

* fit for cluster interface

* fix direction

* add AllowMerge for namespace classifier

* fix direction

* tiny change

* temp commit

* normal priority

* fix score when region size is zero

* clean up

* update kvproto

* fix test

* tiny change

* address comment

* add limit for merge

* add operators in batch

* fix ci

* add metrics and log

* use region instead of direction

* retry more times in test

* extract function

* add admin merge command

* fix

* add random merge scheduler

* check no store

* fix no region

* address comment

* address comment

* fix test

* add region sibling command for pd-ctl

* fix format

* fix test

* skip down peers and pending peers

* skip zero

* address comment

* address comment

* address comment

* fix conflict

schdule: exclude offline store in filter source (tikv#983)

server: increase replica scheduler limit default value (tikv#960)

schedulers: fix hot region distinct score filter (tikv#934) (tikv#939)

simulator: add hot region cases. (tikv#909)

* simulator: add hot region cases.

heartbeat: rebind stream. (tikv#918) (tikv#921)