curvefs/client: fix bug of io hang

doc: add changelog-2.1.md

curvefs/client: fix client release read data cache core dump

curvefs v0.2.0-beta

curve: support block storage open multiple times

support block storage open multiple times
e.g., a single file/image can be mapped multiple times on different hosts by
curve-nbd with extra option `--no-exclusive'

limitations:
1. provided as a shared disk/device WITHOUT data consistent guarantee,
   so it's users' responsibility to deal with this problem.
2. each file/image can only open once on each host.

curve-ops-tools: fix chunkserver ip:port error

docker: update entrypoint.

make io smooth

curve-nbd: change auto mount service to systemd

curve-nbd: change auto mount service to systemd