Merge branch 'rocksdb-log-size-fix' into release-3.1

raft: fixed incorrect observer promotion (#94)

dragonboat: fixed a data race when requesting leadership transfer

dragonboat: get ready for the v3.1 release

dragonboat: bumped version number to v3.0.2

dragonboat: bumped version to v3.0.1

dragonboat: removed the dev version tag

dragonboat: bumped version number to 2.1.7.

dragonboat: bumped version number to 2.1.6.

dragonboat: bumped version number to v2.1.5.