Upgrade to first ABCI++ celestia branch

[evan-forbes]

Currently the app and core utilize the ABCI to communicate. This limits the possible solutions we can pursue for important features such as #455, #454, and #77. While the current implementation of ABCI++ is not entirely stable, we should incorporate it so that we can begin to work on solutions to issues above.

This will involve

• using the master branch (or whatever the abci++ branch ends up being) of celestia-core
• Refactoring the PreprocessTxs method to PrepareProposal