v0.12.2.0

This release implements a hard fork to extend claim expiration. All users must update before 7/9/18.
This release requires a reindex for all users utilizing the RPC command getclaimbyid.

Details of hard fork:

The fork of the mainnet will be on block 400155 (will be around noon EST 7/9/18)

The fork of the testnet will be on block 278160 (already took place on 5/22/18)

Claims made on and after the target hardfork blocks will expire after 2102400 blocks (approximately 10 years assuming block time 2.5 minutes).

Claims that were made before the hardfork and have not expired will have their expiration extended by the difference between the new expiration time and the previous expiration time (456 days).

Claims that already expired will remain expired.

Additional changes
Added a claim index for fast reference by claim ID (#110).
Fixed testnet to redadjust difficulty in case of large hashrate drop (#141)