Changes in synapse v0.28.1 (2018-05-01)

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SECURITY UPDATE

* Clamp the allowed values of event depth received over federation to be
  [0, 2**63 - 1].  This mitigates an attack where malicious events
  injected with depth = 2**63 - 1 render rooms unusable.  Depth is used to
  determine the cosmetic ordering of events within a room, and so the ordering
  of events in such a room will default to using stream_ordering rather than depth
  (topological_ordering).

  This is a temporary solution to mitigate abuse in the wild, whilst a long solution
  is being implemented to improve how the depth parameter is used.

  Full details at
  https://docs.google.com/document/d/1I3fi2S-XnpO45qrpCsowZv8P8dHcNZ4fsBsbOW7KABI/edit#

* Pin Twisted to <18.4 until we stop using the private _OpenSSLECCurve API.