POW Code optimization (about 4% faster)

There are 2 small optimizations that can be made to the pow algorithm to speed it up by a few %

• the first hash is calculated for every nonce, but it only changes once per block. Precalculate it and save one sha256 per nonce.
• don't check if the miner shoud stop every nonce. Once every 50 is plenty.

Changes are only made to:
libraries/plugins/witness/witness.cpp

Here is the diff:
http://hastebin.com/raw/ecifidawiz

And here the full file with changes:
http://hastebin.com/raw/egajuyuguq

just run make after changing this and restart the client.
Like if you like!

It can be combined with the gmp/mpir modification. Btw, compiling libgmp yourself also adds some hashes!