Use uint64 instead of int64 in QGB types

[rach-id]

To avoid confusion between data types, this PR changes int64 to uint64 in the right places

[codecov-commenter]

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[evan-forbes]

the thinking around using int64 is that that is what tendermint uses for height, but I'm fine with keeping everything unsigned

[rach-id]

@evan-forbes we can move everything to be signed to stay consistent with tendermint. Should we ?
This PR aims to have the same type instead of different ones and doing conversions in different places. Either int64 or uint64.

[evan-forbes]

I could be wrong, but I think its arbitrary for our use case since we will have to convert to an evm friendly encoding of a big.Int any way

[evan-forbes]

being consistent makes sense tho, so up to you

[rach-id]

uint64 make more sense for me. I will stick with it for now and in the future, if needed, we can switch again.