Add a pre-generated address to the shared branch

Now wallets starts with a pre generated address on the shared branch. This way when every copayer open the first time the wallet they will find an address instead of an empty list.

Shared branch rationale:
* It's too complicated to agree in one copayer to create the first address and work properly in an asynchronous way. This solution may involve managing the concept of "wallet creator".
* We have it and using it make sense.

Closes #967