Add DPOS rpc endpoints

[pathornteng]

This PR implement 2 rpc endpoints, dpos_state and dpos_list_all_delegations

These endpoints can be called to get dpos state and all delegation at a particular height

http://localhost:46658/query/dpos_state?height=100
http://localhost:46658/query/dpos_list_all_delegations?height=100

• [ ] I added unit tests for any code that added
• [ ] I updated the CHANGELOG.md
• [x] All IP is original and not copied from another source
• [x] I assign all copyright to Loom Network for the code in the pull request