Add a new member exhaustCursorId in DbResponse object to hold the exhaust cursor id information.

receivedCommands() in the command execution layer should return DBResponse with exhaustNS & exhaustCursorId members populated from response data (which is built by execCommandDatabase() and stored in replyBuilder->getBodyBuilder()) cursor.ns and cursor.id fields respectively. The values should be populated before serializing the response data and sending it to network layer. The values gets populated only if the following conditions are satisfied:

• Request message has OpMsg::kExhaustSupported flag set.
• Response data has “ok” field set to 1 (no error) and contains “cursor” object field.