Check Compatibilty with Tendermint and decide if to use int64 or uint64 for block height

[nashqueue]

Rollmint has an inconsistency what type it uses for block height.
Examples for int64, Examples for unit64 .

It is caused by "compatibility" issues with Tendermint. We need to decide which type to use.

Discussion originally started in #665 (comment)_

[tzdybal]

Uh, oh, I duplicated it in #688, but there's also a bit wider context.