Support forward upgrade

[julienrbrt]

Currently the multiplexer is working well for syncing for genesis with previous heights.
However we need to make it work with future heights as well.

In theory, it is already supported, as the current app prior the the upgrade could already be added to the multiplexer, leaving the new app as the latest app. However, for that app switch, we shouldn't rely on a fixed height, but instead a dynamic one.

To keep the multiplexer not tied to any specific module, we should add a boolean to Version to tell the multiplexer to determine the app version change from abci directly.