StreamCompletionSignal options should match spec

[lexs]

Filing so I don't forget.

We never use other values than GRACEFUL and ERROR from StreamCompletionSignal.
There is an assert here that can never trigger because of this: https://github.com/ReactiveSocket/reactivesocket-cpp/blob/master/src/ConnectionAutomaton.cpp#L105

For reference the current values of the enum are:

enum class StreamCompletionSignal {
  GRACEFUL,
  ERROR,
  INVALID_SETUP,
  UNSUPPORTED_SETUP,
  REJECTED_SETUP,
  CONNECTION_ERROR,
  CONNECTION_END,
};

[benjchristensen]

A stream can be terminated in these ways:

• closed (normal successful termination)
• APPLICATION_ERROR (normal application failure termination)
• REJECTED
• CANCELED (unlikely that we'll use this)
• INVALID

See https://github.com/rsocket/rsocket/blob/master/Protocol.md#error-codes for the error types and descriptions. See https://github.com/rsocket/rsocket/blob/master/Protocol.md#stream-sequences-and-lifetimes for each stream type and possible terminations. For example:

• PAYLOAD with COMPLETE
• ERROR[APPLICATION_ERROR|REJECTED|CANCELED|INVALID]

[benjchristensen]

Thus:

enum class StreamCompletionSignal {
  COMPLETE,
  APPLICATION_ERROR,
  REJECTED,
  CANCELED,
  INVALID,
};