Refactor away BlockHeight(0)

[james-chf]

In Tendermint, the earliest possible block in a chain (i.e. genesis block) has height 1. Currently our BlockHeight type permits BlockHeight(0) which isn't a valid block height and BlockHeight::default() == BlockHeight(0). The first proposed block will be at least BlockHeight(1). We should avoid BlockHeight(0) in our code.

We could define BlockHeight something like:

struct BlockHeight(NonZeroU64)

It could be worth having different types for proposed block heights and committed block heights. e.g. shell.storage.last_height represents the last committed block height and so might be of type Option<CommittedBlockHeight>. Often when dealing with ABCI(++) logic, we are interested in the current ProposedBlockHeight rather than the last CommittedBlockHeight, so our functions could explicitly work with ProposedBlockHeights only where appropriate.

[tzemanovic]

duplicate of #648