Storing and hashing old root hashes

[Jasleen1]

We likely want to have a way to access old root node hashes, even after a tree is updated. For example, at the moment the tests which require multiple root hashes need to obtain the hash for a particular publish before the next call to publish. Additionally, we probably want to support chaining the hashes together, although this is up for debate.

[Ercan Ozturk]

@Jasleen1, I thought this was done but I see that the epoch used here is not right. Could you take a look?