Decide on type of height related fields

[tzdybal]

Original implementation used uint64 to represent any type of height. This seems to be the most appropriate type.
The convention in Tendermint/Cosmos-SDK is to use int64. The same for go-header library.

We need to decide what type should be used and change the code accordingly.

[Wondertan]

Note that go-header’s int64 usage is not a requirement, and we can change it if needed.

[Manav-Aggarwal]

Duplicate of #1176