Implement stake kernel mining as a separate function

This change allows us to reduce amount of intermediatte operations, such as header hashing or modifier calculation. As the result, we've got ~20x drop of CPU load and 60x boost of kernel scanning speed.
Coins selection fix
Coins metadata load fixes

* Don't load coin and meta if we unable to calculate suitable stake modifier for this coin;
* Move definition of MapMeta to CWallet class;
* Use MapMeta for stake weight calculation.
RPC: modify mergecoins

* Parameter set changes (implement minimum coin credit and use output credit as a maximum value of coin credit);
* Handle of maximum and minimum coin credit while selecting available inputs.
Getstakeweight: load coins metadata if there are no items loaded
Merge pull request #30 from CryptoManiac/master

Coin operation updates