Orbitcoin Green Stake

A subsystem designed to improve staking performance and efficiency.

- Caching of block hashes, stake modifiers and stake inputs;
- User accessible split and combine controls for stake inputs
  (-stakesplit and -stakecombine);
- User changeable min. input amount for staking (-minstakinginput);
- Performance counters to monitor caching internals (RPC getcounters).

Other minor changes.