fix InitFromSeed concurrency bug
This PR fixes https://github.com/NebulousLabs/Sia-UI/issues/583 and https://github.com/NebulousLabs/Sia-UI/issues/581, the wallet was not holding a lock during the duration of the InitFromSeed call, causing users to unlock their wallets while the seed sweeper was still running, resulting in an incorrect seed progress value.