* core/state/snapshot: add diskRoot function
* core/state/snapshot: disable iteration if the snapshot is generating
* core/state/snapshot: simplify the function
* core/state: panic for undefined layer
* core/state/snapshot: implement storage iterator
* core/state/snapshot, tests: implement helper function
* core/state/snapshot: fix storage issue
If an account is deleted in the tx_1 but recreated in the tx_2,
the it can happen that in this diff layer, both destructedSet
and storageData records this account. In this case, the storage
iterator should be able to iterate the slots belong to new account
but disable further iteration in deeper layers(belong to old account)
* core/state/snapshot: address peter and martin's comment
* core/state: address comments
* core/state/snapshot: fix test
