satellite/gc: Init map with an aprox size

Because the PieceTracker receives a piece count per nodes which is an
approximation of the number of nodes that they are going to be reported
by the metainfo loop so we can use as a good guess of the map's size and
initialized with it.

Change-Id: I644db40926c03e4c457457fb41d2ec1da059cea6

satellite/gc/piecetracker.go

@@ -38,7 +38,7 @@ func NewPieceTracker(log *zap.Logger, config Config, pieceCounts map[storj.NodeI
 		creationDate: time.Now().UTC(),
 		pieceCounts:  pieceCounts,
 
-		retainInfos: make(map[storj.NodeID]*RetainInfo),
+		retainInfos: make(map[storj.NodeID]*RetainInfo, len(pieceCounts)),
 	}
 }