Add DPOS consensus

[panda-sandeep]

Find a way to incorporate DPOS consensus into the ABCI server. One of the ways to achieve this can be something like this:

• Add a "voting" transaction and let people cast their votes in favor of specific validators
• The global state can store a sorted list of top 21 validators
• The EndBlock callback can be used to update the list of validators so that this list changes each time a vote is cast.

Resources:

• DPOS: https://bitshares.org/technology/delegated-proof-of-stake-consensus/
• How Tendermint works: http://tendermint.readthedocs.io/en/master/introduction.html