Preparation for bitcoinj 0.12.3

[Andreas Schildbach]

I started putting together commits for bitcoinj 0.12.3:

https://github.com/schildbach/bitcoinj/commits/release-0.12

Basically it's performance improvements, logging changes and some fixes.

But there is also two added API methods:

- Wallet.reset() to prepare the wallet for a blockchain replay
- Wallet.getIssuedReceiveKeys()/getIssuesReceiveAddresses() for knowing
the derived keys.addresses.

Is there anything else you need? I plan to backport some commits from
Mike's wallet-output-tracking branch once that is merged and release
soon after.