Encode heights used as keys in sled-based lightstore in big-endian representation

[romac]

According to the sled documentation [1] and example [2], one needs to represent the keys in big-endian form for sled's iterators and ordered operations to work properly.

Our light client might currently be misbehaving when more than 255 blocks were stored in the database.

[1] https://github.com/spacejam/sled#a-note-on-lexicographic-ordering-and-endianness
[2] https://github.com/spacejam/sled/blob/aa3fcf37d844e2d1382e0a58267dc3b3ae3b25aa/examples/structured.rs#L23-L30