Add setIdentity method

[oed]

Feature request

Right now the identity of the store is set when it's opened. It would be great if it was possible to change the identity after the store has been initialized. A use case for this might be if you want to sync the data of the store before the user has been authenticated, if so a temporary identity could be used until that happens.

How it would work

Add a new method called setIdentity. This method would change this.identity of the store to the identity that is passed to the method. It would also update the identity in the ipfs-log so a change is needed there as well.