New Wallet Database Format

Consider saving witness related data to a file, e.g. witness.dat instead of storing it in wallet.dat.

• Separation of data, to isolate and only store keys in wallet.dat.
• Performance by avoiding contention between different subsystems trying to write to wallet.dat
• Avoid frequent writing of data to wallet.dat on block rescan and reindex.
• Reduce risk that when a system crash occurs, wallet.dat is being written to

Related, see zcash/zcash#1749