Prevent single file storage from performing unnecessary N^2 loop

For single file storage we wrap the logic of get_multi with the
at_once context manager so that `self.list()` (which is called by
`self.get()`) actually caches the items rather than re-computing
them at every call.

This should largely mitigate the performance issue describe at
https://github.com/pimutils/vdirsyncer/issues/818 . The issue
discussion also contains more background about this patch.

vdirsyncer/storage/singlefile.py

@@ -4,6 +4,7 @@ import functools
 import glob
 import logging
 import os
+from typing import Iterable
 
 from atomicwrites import atomic_write
 
@@ -11,6 +12,7 @@ from .. import exceptions
 from ..utils import checkfile
 from ..utils import expand_path
 from ..utils import get_etag_from_file
+from ..utils import uniq
 from ..vobject import Item
 from ..vobject import join_collection
 from ..vobject import split_collection
@@ -132,6 +134,12 @@ class SingleFileStorage(Storage):
         except KeyError:
             raise exceptions.NotFoundError(href)
 
+    async def get_multi(self, hrefs: Iterable[str]):
+        async with self.at_once():
+            for href in uniq(hrefs):
+                item, etag = await self.get(href)
+                yield href, item, etag
+
     @_writing_op
     async def upload(self, item):
         href = item.ident