Clear the blockchain file (as opposed to delete it) in preparation for restore

[oscarguindzberg]

This is an alternative solution to #1695

[schildbach]

Sounds like a good idea to me!

[schildbach]

If that solves your problem, I like it more than #1695.

[oscarguindzberg]

I just:

• Removed WalletAppKit.provideBlockStore() and modified WalletAppKit's vStore instance variable type to stress it is of SPVBlockStore type: WalletAppKit won't work with another store type. The "overridable" solution was half-way done. Overriding provideBlockStore() to return any of the other BlockStore implementations included in bitcoinj would make the system fail on restore from seed/key.
• Rebased from master.
• Removed "[WIP]" label from the title.

[oscarguindzberg]

@schildbach don't forget about this PR :)

[schildbach]

Thanks for the reminder! I just merged.