Retry PUT 409 errors - delete and re-create event

Attempt to address #963

I would appreciate a code review here - there might be a more elegant way to address this problem, but it works for my (very limited) use case.

vdirsyncer/storage/dav.py

@@ -587,7 +587,16 @@ async def _put(self, href, item, etag):
     async def update(self, href, item, etag):
         if etag is None:
             raise ValueError("etag must be given and must not be None.")
-        href, etag = await self._put(self._normalize_href(href), item, etag)
+        try:
+            href, etag = await self._put(self._normalize_href(href), item, etag)
+        except aiohttp.ClientResponseError as e:
+            if e.status == 409:
+                dav_logger.debug("Conflict, will delete old event and recreate it.")
+                await self.delete(self._normalize_href(href), None)
+                dav_logger.debug("Now trying again")
+                href, etag = await self._put(self._normalize_href(href), item, None)
+            else:
+                raise e
         return etag
 
     async def upload(self, item: Item):