New rule modifier, improved backtesting

Changes

• Added new rule modifier: MaxPairs (limit the number of trading pairs based on the market conditions, can be used with a pair-specific condition)
• Added new experimental backtesting settings: TradingSpeedEasing, TradingRulesSpeedEasing & SignalRulesSpeedEasing. These can be used to throttle some of the service tasks that are causing too much lag. Use sparingly.
• Minor improvements to backtesting
• General code refactoring

Upgrading from v0.9.9.7

• Optionally add the new backtesting settings to backtesting.json (they default to 0, which is a standard behaviour)
• Replace the bin folder