feat(gov): software upgrade governance proposal support

[SebastianElvis]

Overview

Software upgrade is the process of upgrading the node software version in the middle of the execution of the blockchain. In Cosmos SDK chains, this is achieved via a software upgrade governance proposal: one submits a proposal suggesting that the chain upgrades to a given version at a given height. If the proposal has passed, then upon this given height, the chain will halt. Only after the supermajority of validators have restarted with the given version, the chain will resume execution.

The upgrade can be breaking in the sense that the new DB schema is not compatible with the old one, e.g., a module is added/removed, DB schema has changed, etc.. In this case, the developer needs to write a migration function defining how the state under the old schema can be applied to the new schema. The migration function is versatile in the sense that one can manipulate state at wish.

As an example, Osmosis defines such callback function for all software upgrades, and this one tops up OSMO tokens to a number of accounts given here. Agoric has something similar here.

Tasks

This issue defines the task on adding software upgrade support to Babylon. We take Osmosis as the reference implementation. This includes a set of steps:

• separating the keepers into a new struct #699
• implementing the relevant data structures for streamlining the definition of software upgrades #700
• implementing the test framework and a vanilla test #701
• fix: execute PreBlocker for upgrade module #707