Upstream changes tendermint core

[mattkanwisher]

We need to upstream following

1. Reactor support - https://github.com/tendermint/tendermint/pull/3730
2. Iavl changes https://github.com/tendermint/iavl/pull/138
3. Exposing the DB interface better https://github.com/loomnetwork/tendermint/commit/cd89d233c49084a398204be71844761b41a74eb6
4. We both changed how Leveldb metrics are exposed, lets see if we can commonize or upstream this if ours is more feature rich

[enlight]

1. They've made some tweak to the custom reactor API in https://github.com/tendermint/tendermint/pull/3771 - we should merge those changes back to our fork at some point.
2. They've changed a bunch of stuff, still not merged though https://github.com/tendermint/iavl/pull/158
3. Need to track down what happened with this one.