Listkeys (#336)

* allow other tasks to run fileClient.listKeys

* avoid blocking and list_keys

* added handlers for timeout error

* fix flake8 errors

* fix modulo check

* fix for modulo

hsds/util/fileClient.py

@@ -367,7 +367,7 @@ async def list_keys(
                     files.append(filepath)
                     if limit and len(files) >= limit:
                         break
-                    if len(files) % 1000:
+                    if len(files) % 1000 == 0:
                         await asyncio.sleep(0)
 
         # use a dictionary to hold return values if stats are needed