Reset peakZ on new print

(cherry picked from commit 83a864d)

octoprint/printer.py

@@ -56,7 +56,7 @@ class Printer():
 
 		self._currentZ = None
 
-		self.peakZ = -1
+		self._peakZ = -1
 		self._progress = None
 		self._printTime = None
 		self._printTimeLeft = None
@@ -185,6 +185,7 @@ class Printer():
 			return
 
 		self._setCurrentZ(None)
+		self._peakZ = -1
 		self._comm.startPrint()
 
 	def togglePausePrint(self):
@@ -396,8 +397,8 @@ class Printer():
 		oldZ = self._currentZ
 		# only do this if we hit a new Z peak level.  Some slicers do a Z-lift when retracting / moving without printing 
 		# and some do anti-backlash up-then-down movement when advancing layers
-		if newZ > self.peakZ:
-			self.peakZ = newZ
+		if newZ > self._peakZ:
+			self._peakZ = newZ
 			eventManager().fire("ZChange", newZ)
 			
 		self._setCurrentZ(newZ)
@@ -615,7 +616,6 @@ class StateMonitor(object):
 		self._gcodeData = None
 		self._sdUploadData = None
 		self._currentZ = None
-		self._peakZ = -1
 		self._progress = None
 
 		self._changeEvent = threading.Event()