Standardize dircache timeout

[TomAugspurger]

Currently gcsfs implements a cache_timeout and s3fs might want one too (fsspec/s3fs#271, fsspec/s3fs#253). So maybe we should just add it to AbstractFileSystem.

Proposal

Adds a cache_timeout to AbstractFileSystem.__init__. Taken from the gcsfs docs

    cache_timeout: float, optional
        Cache expiration time in seconds for object metadata cache.
        Set cache_timeout <= 0 for no caching, None for no cache expiration.

As for the default, I think None makes the most sense for now since that's what gcsfs does. We can explore deprecating None in favor of something else (I think 0 for no caching) down the road.

One question: is this timeout per-key, per-directory, or global to the filesystem instance? In the gcsfs docs, we state

GCSFileSystem maintains a per-implied-directory cache of object listings and
    fulfills all object information and listing requests from cache. 

But from what I can tell, the cache is actually per object, not per-directory.

    @_tracemethod
    def _list_objects(self, path):
        ...
        listing = self._do_list_objects(path)
        retrieved_time = time.time()

        self._listing_cache[path] = (retrieved_time, listing)
        return listing

and

    @_tracemethod
    def _maybe_get_cached_listing(self, path):
        ...
        if path in self._listing_cache:
            retrieved_time, listing = self._listing_cache[path]
            cache_age = time.time() - retrieved_time
            if self.cache_timeout is not None and cache_age > self.cache_timeout:
                logger.debug(
                    "expired cache path: %s retrieved_time: %.3f cache_age: "
                    "%.3f cache_timeout: %.3f",
                    path,
                    retrieved_time,
                    cache_age,
                    self.cache_timeout,
                )
                del self._listing_cache[path]
                return None

            return listing

        return None

[TomAugspurger]

cc @martindurant

[martindurant]

Should be per listing, i.e., a directory of stuff. A listing could be a single key-item in the case that the parent hadn't been previously listed.