arrow

ARROW-3116: [Plasma] Add "ls" to object store

This adds plasma_client.list to the plasma client API.

It can be used like so:

```python
  import pyarrow.plasma as plasma
  import time

  client = plasma.connect("/tmp/plasma", "", 0)

  client.put("hello, world")
  # Sleep a little so we get different creation times
  time.sleep(2)
  client.put("another object")
  # Create an object that is not sealed yet
  object_id = plasma.ObjectID.from_random()
  client.create(object_id, 100)
  print(client.list())

  >>> {ObjectID(4cba8f80c54c6d265b46c2cdfcee6e32348b12be): {'construct_duration': 0,
  >>>  'create_time': 1535223642,
  >>>  'data_size': 460,
  >>>  'metadata_size': 0,
  >>>  'ref_count': 0,
  >>>  'state': 'sealed'},
  >>> ObjectID(a7598230b0c26464c9d9c99ae14773ee81485428): {'construct_duration': 0,
  >>>  'create_time': 1535223644,
  >>>  'data_size': 460,
  >>>  'metadata_size': 0,
  >>>  'ref_count': 0,
  >>>  'state': 'sealed'},
  >>> ObjectID(e603ab0c92098ebf08f90bfcea33ff98f6476870): {'construct_duration': -1,
  >>>  'create_time': 1535223644,
  >>>  'data_size': 100,
  >>>  'metadata_size': 0,
  >>>  'ref_count': 1,
  >>>  'state': 'created'}}
```

Author: Philipp Moritz <pcmoritz@gmail.com>

Closes #2470 from pcmoritz/plasma-list and squashes the following commits:

5ff4e355 <Philipp Moritz> fix
32c36a75 <Philipp Moritz> minor fix
a58db5bc <Philipp Moritz> add more documentation
1f0c91de <Philipp Moritz> fix
1f384ee9 <Philipp Moritz> add documentation
0958e4d1 <Philipp Moritz> merge
ce122957 <Philipp Moritz> add test
a6ba6f6f <Philipp Moritz> fix
62772a87 <Philipp Moritz> add timestamp
bb8d52f0 <Philipp Moritz> fix
6572943d <Philipp Moritz> fix
d343d4fb <Philipp Moritz> linting
96040ad4 <Philipp Moritz> cleanups
6f29cec0 <Philipp Moritz> fix test
79a2b549 <Philipp Moritz> make variables unique
93a9ca53 <Philipp Moritz> fix linting
f7f36068 <Philipp Moritz> add test cases and fixes
189928be <Philipp Moritz> add more fields
deb06b42 <Philipp Moritz> get list working
ac1fae62 <Philipp Moritz> update
a07db83b <Philipp Moritz> add list command to store
400d326f <Philipp Moritz> create object list