Relation.getPlayers should return a promise

[flyingsilverfin]

Problem to Solve

We expect all networked Concept API calls to be async Promises or Streams - however, we find that Relation.getPlayers returns a Map in a blocking style in both Python and Java.

Proposed Solution

Fix Python and Java to return promises for Relation.getPlayers.

We should also validate that all Concept API calls that take transactions are returning Promises or Streams.