Only allow users to transition using the latest history tree root

[vimwitch]

Users could execute a UST using an old history tree root. This could reveal what epoch they are transitioning from. A malicious application could do this without a users knowledge to try to de-anonymize them. Users could also conspire to transition using old history tree roots to reveal users that transition using a newer root.

To solve this we should require the UST proof to use the latest history tree root. This also gets rid of the historyTreeRoots mapping and an SSTORE operation.