0.17.4

CHANGES:

• tls: handle half-closed connection properly: a received CLOSE_NOTIFY does not
  lead to a CLOSE_NOTIFY to be sent (a send_close_notify sends it explicitly)
  (#488 @hannesm)
• tls: modify return type of handle_tls - the Alert is now in the right hand
  side, and `Eof is explicit in the second part of the tuple
  (#488 @hannesm)
• tls: remove can_handle_appdata, the function handshake_in_progress is
  available (#488 @hannesm)
• tls-mirage: avoid exceptions in reneg and rekey (#487 @hannesm)
• tls: remove HEARTBEAT decoding - HEARTBEAT was never supported in this
  library, the decoder was superfluous (#487 @hannesm)
• tls-mirage: provide underlying : flow -> FLOW.flow (#487 @hannesm,
  fixes #425 @dinosaure)
• tls-mirage: implement mirage-flow 4 API (val shutdown) (#488 @hannesm)
• tls-eio: adapt to half-closed connections (#488 @talex5)
• tls-eio: implement Eio.Resource.Close (#489 @paurkedal, reviewed by @talex5)