all: recommit genesis state for 0 rewinding target

[rjl493456442]

Geth will try to rewind its head to a lower point if the corresponding state is not found.
But in order to avoid the unnecessary disk load, if the block is lower than pivot point, then
the state won't be checked, instead genesis block will be used as the destination directly.

In the current code base, the assumption is held that genesis state is always existent in the
disk. However, in the new state scheme(path based), state will always be overwritten and
genesis state can be gone in this case.

This PR wants to get rid of the assumption, but recommit the genesis state instead. theoretically
it should make no difference right now, but this logic can be merged first before the big state PR.

Warning: most of the changes are in the test!