Memory iterator.Seek probably needs to return to previous location on failed Seek

[AndrewSisley]

If a failed Seek is performed, e.g. to an out of bounds value, it probably needs to reset to it's prior state and return false.

I am unsure if we have any tests yet for this, make sure the memory store behaviour matches badger.

It might be that this doesn't really make sense or can be postponed with our interface.

Spotted by Fred.