Segwit, sign mixed inputs, migrate to Bouncy Castle, and 2x seed modifications.

This version of bitcoinj was put together for use in Samourai Wallet and Sentinel as it enables signing of mixed inputs (P2PKH and P2SH-P2WPKH). It includes code from the SegWit branch of bitcoinj and GreenBits version of same. It also includes changes to remove Segwit2X DNS seeds as well as all Spongy Castle dependancies (it is 100% Bouncy Castle).