Adds dask lock capability for backend writes

This fixes an error on an asynchronous write for `to_netcdf`
resulting in an `dask.async.RuntimeError: NetCDF: HDF error`

Resolves issue pydata#793
following dask improvement at dask/dask#1053
following advice of @shoyer.

Author: pwolfram

xarray/backends/common.py

@@ -161,8 +161,12 @@ def add(self, source, target):
 
     def sync(self):
         if self.sources:
-            import dask.array as da
-            da.store(self.sources, self.targets)
+            import dask, dask.array as da
+            import threading
+            if dask.__version__ > '0.8.1':
+                da.store(self.sources, self.targets, lock=threading.Lock())
+            else:
+                da.store(self.sources, self.targets)
             self.sources = []
             self.targets = []