* reserve colateral keey only if needed
reserve colateral keey only if needed -> fix losing one key on every
subsequent CreateDonominate
* Adding KeyHolder and KeyHolderStorage
Adding KeyHolder and KeyHolderStorage to store keys used in PrivateSend
until status of mixing is known.
* Removing ClearOn... methods. Instead of calling directly KeepAll/ReplaceAll.
* remove usage of shared_ptr
* use unique_ptr
* follow naming convention
* use CKeyHolderStorage in CreateDenominated too
* adjust log messages
* return keys on POOL_STATE_ERROR, keep keys on POOL_STATE_SUCCESS
* Disable copy/assign and allow move
* Use keyHolderStorageDenom for collateral output in CreateDenominated
