Add cache timeout and directory cache

[TomAugspurger]

Closes #215

WIP for now, I need to incorporate #215 (comment).

[TomAugspurger]

@martindurant Is there a standard schema for what entries in DirCache should look like? I see that github and ftp use them. In fsspec/spec.py, it's only used in _ls_from_cache, which seems to expect something like

{
    "path": {
        "name": name,
    }
} 

Standardizing the structure here would be good (maybe in a dataclass?), but I'm not sure what all to include yet.

[TomAugspurger]

For ftp, elements of DirCache are List[Tuple[str, Dict]], where the first item of the tuple is the path of the element, and the dict has a schema like modify, perm, size, file, unique, name.

(Pdb) pp out[:2]
[('__init__.py',
  {'modify': '20190813183127',
   'name': '/__init__.py',
   'perm': 'r',
   'size': 0,
   'type': 'file',
   'unique': '1000004g2058413d7'}),
 ('__pycache__',
  {'modify': '20191127162327',
   'name': '/__pycache__',
   'perm': 'el',
   'size': 0,
   'type': 'dir',
   'unique': '1000004g206bad42b'})]

those are all the files under the path (the key).

For github we just have a List[Dict], and the keys in the dict are name, mode, type, size, sha.

These are inconsistent. At the moment, I'm leaning toward a namedtuple structure like

CacheItem = namedtuple("CacheItem", ["name", "details"])

where name is a string, and details is a dict with anything. Hopefully that will suffice.

[martindurant]

The canonical structure should be:

{'cached_path`: [
    {"name": 'file_path", 
     "size": 10,
     "type": "file"},
    ...
   ]
}

The FTP case is clearly based on the output of the client library, and ought to be processed into canonical form, as it done for s3, gcs...

[martindurant]

i.e., the key is the path that we did a listing for
(but I'm fine with the inner structure being dict-like too, so we can find an entry quickly; however, it may be possible on, e.g., s3, that two identical names exist, one as a prefix and one as a file)

[martindurant]

This still looks useful to me

[martindurant]

Superceded by #243