Improvements to upgrade module

[aaronc]

Motivation: cosmos sometimes pushes breaking changes to the genesis config that would force us to recreate our testnet. We shouldn't be lazy and tear down our testnet every time something like this happens and we actually need a more graceful way to deal with it in production.

• the upgrade keeper should allow for some "on upgrade" code to be called whenever a chain restarts after a scheduled upgrade. This upgrade code can perform state changes necessary to support the upgrade before any other transactions are processed (so it should happen in the BeginBlocker)
• allow upgrades based on time or block height
• make the module as simple as possible but not simpler
• thorough docs, run golintand make sure there are tutorial docs
• thorough test coverage
• adapt Postgres indexer package to properly support migrations and integrate them with upgrade module for testnet
• add cli query and REST support, and possibly a cli tx cmd helper