add return value for RequestChannel::getNextInteractionId()
add return value for RequestChannel::getNextInteractionId()

Summary:
When this code sync to ovrsource, it's complaining for not having return value in RequestChannel::getNextInteractionId() function, so add a return value here
failing job: https://www.internalfb.com/intern/sandcastle/job/27021598070424290/

Reviewed By: iahs

Differential Revision: D22861608

fbshipit-source-id: b69a50939ba22b26f53a18fb357ae381c4759cb2

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift