Note that this PR breaks the PlasmaClient API (which is still unstable at this point, so this is acceptable). It renames PlasmaClient.get to PlasmaClient.get_buffers and introduces two new functions, PlasmaClient.put and PlasmaClient.get which can put Python objects into the object store and provide access to their content. The old get was renamed to get_buffers because most users will want to use the new get method and therefore it should have the more concise name.

There is some freedom in designing the API; I tried to make it so there is a unified API between getting one and multiple objects (the latter is supported to limit the number of IPC roundtrips with the plasma store when we get many small objects). I also introduced a special object that is returned if one of the objects was not available within the timeout. We could use "None" here, but then it would be hard to distinguish between getting a "None" object and a timeout.