Strategy: refactoring

- rm PositionProbabilities()
- add MaxTradeDuration()

strategy.go

@@ -1,12 +1,14 @@
 package sentio
 
+import "time"
+
 type Strategy interface {
 	Name() string
 
 	Model() string
 	MarketId() string
 	PositionSymbols() map[Side]string
-	PositionProbabilities() map[Side]float32
+	MaxTradeDuration() time.Duration
 	Interval() uint8
 
 	Handle(market Market, turn *Turn) error