[Plasma] Pull SerializationContext through PlasmaClient put and get

[asfimport]

Expose the SerializationContext through PlasmaClient.put and get.

I'd be good to get this into 0.7 so Ray can use that version.

Reporter: Philipp Moritz / @pcmoritz

_{Note: This issue was originally created as ARROW-1438. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 1019
#1019