cherry-pick some bug fixes from master. (tikv#895)

api: support add region and hot region scheduler. (tikv#866)

Makefile: use git describe to assign PDReleaseVersion (tikv#848)

*: update version and status badge. (tikv#802)

*: bump version and add status badge. (tikv#801)

*: refactor storeInfo. (tikv#712)

api: support delete member by id. (tikv#695)

pd-client: fix a data race in pd client (tikv#660)

Merge pull request tikv#542 from pingcap/tennix/disable-cgo

disable cgo

*: update vendor. (tikv#486)