Iteration over FastCache

[ValarDragon]

This should hopefully be simple:

in immutable_tree.go we should change the Iterate function to check if

t.version == t.ndb.GetLatestVersion()

If so, then we iterate over the underlying node db's keys directly. This involves looking at the key formats, but should just be a direct call on DB iterator function: https://github.com/tendermint/tm-db/blob/master/types.go#L50

We need to make sure the iterator tests test both iterating on the latest version and old versions. Also should sanity check if this is getting called in the SDK once its done. (Can do this later)