AvailableCoins: acquire cs_main mutex

It's required when called from WalletModel
Merge pull request #4371

ea3acaf AvailableCoins: acquire cs_main mutex (Pavel Vasin)
Make CBlockIndex a subclass of CBlockHeader
Unify CBlockHeader::hashPrevBlock and CDiskBlockIndex::hashPrev
Make GetBlockHash deprecated
Remove GetBlockHeader()
CBlockIndex* CDiskBlockIndex::CreateIndex() instead of manual mapping