Rarity Map Performance fix

Do not calculate the rarest piece unless it is needed.
It is an expensive operation.

lib/rarity-map.js

@@ -10,21 +10,21 @@ class RarityMap {
     this._pieces = new Array(this._numPieces)
 
     this._onWire = wire => {
-      this.recalculate()
+      this._dirty = true
       this._initWire(wire)
     }
     this._onWireHave = index => {
       this._pieces[index] += 1
     }
     this._onWireBitfield = () => {
-      this.recalculate()
+      this._dirty = true
     }
 
     this._torrent.wires.forEach(wire => {
       this._initWire(wire)
     })
     this._torrent.on('wire', this._onWire)
-    this.recalculate()
+    this._dirty = true
   }
 
   /**
@@ -35,6 +35,11 @@ class RarityMap {
    * @return {number} index of rarest piece, or -1
    */
   getRarestPiece (pieceFilterFunc) {
+    if (this._dirty) {
+      this._dirty = false
+      this.recalculate()
+    }
+
     let candidates = []
     let min = Infinity