fix: empty shard need ensure idle state every coordinate period

Merge branch 'master' of https://github.com/tkestack/kvass

# Conflicts:
#	examples/prometheus-rep-0.yaml

fix: coordinator min shard chaos with max shard

fix: sidecar panic when scrape failed (tkestack#32)

Co-authored-by: RayHuangCN <[email protected]>

feat: disable scaling down by default

fix: scrape timeout message of targets list

fix: change workflow go version to 1.15

fix: Dockerfile and Makefile

style: clean code