[Plasma] Create Cython client library for Plasma

[asfimport]

This would address the point raised in #742 by @wesm and replace the current C Python API with Cython.

Advantages:

• More uniform with the rest of Arrow (especially pyarrow)
• can share code with pyarrow
• makes error handling more uniform
• easier to program than the C Python API

As part of this, we would also fix the visibility of symbols in the plasma library and port the python tests we have for plasma (which are currently in the Ray repo) to this repo.

Reporter: Philipp Moritz / @pcmoritz
Assignee: Philipp Moritz / @pcmoritz

_{Note: This issue was originally created as ARROW-1149. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 797
#797