Investigate making lightwalletd's data structures immutable

[LarryRuane]

Adopting this advice may make lightwalletd more solid, could possibly fix (currently-unknown) memory-use bugs and prevent future bugs, and could improve security (reduce memory-related attacks):

https://levelup.gitconnected.com/building-immutable-data-structures-in-go-56a1068c76b2

Probably should at least investigate. I have no idea how much work this would be.

I remember once we ran into a bug that this would have prevented, which was that the bytes of a block prevhash were reversed for display purposes (we always display block hashes big-endian, but they're stored little-endian), but instead of modifying a copy, we modified the actual stored hash, and that broke code that used that hash later. (The fix was to make a copy before reversing the bytes.)