Changes list:
- Refactoring of Proof-of-Stake checkings code;
- Remove some redundant messages from debug.log output;
- Make stake related DoS checkings harder;
- Database code is updated, direct CCoins links approach is implemented to decrease count of worthless flush and copy events;
- Initial block download optimization, transaction hashes are cached now to make blocks validation faster;
- Multiple CChainDB instantiations were replaced with a single CBlockTreeDB instance;
- CCoinsDB and CBlockTreeDB are updated to use LevelDB storage;
- gettxout and gettxoutsetinfo RPC calls added;
- getblock and getblockbynumber RPC calls are now providing generator public key for proof-of-stake blocks.
