Merge pull request #157 from domob1812/cache-ecdsa

Cache ECDSA signature verifications.
Add CTxIndex::GetContainingBlock from Huntercoin.

Port over a refactoring from Huntercoin that replaces
GetDepthInMainChain by functions to also get the height directly.
Also cache full txPrev in CTxIn.

Store a copy of the previous tx in CTxIn when we have loaded it from
disk already.
Use cached CTxIn::txPrev for CreateNewBlock.

Don't load the prev tx in CreateNewBlock when we have a cached one.
Still accesses the disk for GetDepthInMainChain.
Cache also prevout height for CreateNewBlock.

Add prevHeight to CTxIn in-memory cache.  This should help to further
speed up CreateNewBlock.