dragonboat: bumped version number to v2.1.5.

Merge branch 'use-regular-delete-in-rocksdb' into release-2.1

Merge branch 'restore-remote-fix' into release-2.1

dragonboat: bumped version number to v2.1.2.

dragonboat: bumped version number to v2.1.1

Updated version number to 2.1.0 released.

Bumped version number to 2.0.1

Added gogo protobuf's copyright info.