multi: replace usage of fastsha256 with crypto/sha256

This commit removes all instances of the fastsha256 library and
replaces it with the sha256 library in the standard library. This
change should see a number of performance improvements as the standard
library has highly optimized assembly instructions with use vectorized
instructions as the platform supports.

glide.yaml

@@ -3,7 +3,6 @@ import:
 - package: github.com/boltdb/bolt
   version: ^1.2.1
 - package: github.com/btcsuite/btclog
-- package: github.com/btcsuite/fastsha256
 - package: github.com/btcsuite/go-flags
 - package: github.com/btcsuite/seelog
   version: ^2.1.0