Currently, there are multiple places we can close a session, this makes
the close code path hard to follow. Remove the call to maybe_completed
in follower_start_sent to simplify closing a bit.
- stream_session::follower_start_sent -> maybe_completed()
- stream_session::receive_task_completed -> maybe_completed()
- stream_session::transfer_task_completed -> maybe_completed()
- on receive of the COMPLETE_MESSAGE -> complete()
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