clear alexa session attributes on InOneGoMusicPlayback intent and new session

[hyzhak]

Unlike google action, Alexa skill doesn't have a long running session, which ends with song playback.
But sure, we still need to remember user's dialog context even on song's playback. So we use persistent attributes here.
But we face another problem - we should clear manually psedo-session data.

• we does it for request.type == 'LaunchRequest'
• but we can't catch complex wake up calls like:

Alexa, ask internet archive to play jazz

because from we receive request similar to which we would get in the middle of discussion:

[USER] 78s
[USER] play jazz

except flag session: true:

{
	"version": "1.0",
	"session": {
		"new": true,
		"sessionId": "...",
		"application": {
			"applicationId": "...."
		},
		"user": {
			"userId": "...."
		}
	},
	"context": {
		"AudioPlayer": {
			"playerActivity": "IDLE"
		},
		"Display": {},
		"System": {
			"application": {
				"applicationId": "..."
			},
			"user": {
				"userId": "...."
			},
			"device": {
				"deviceId": "....",
				"supportedInterfaces": {
					"AudioPlayer": {},
					"Display": {
						"templateVersion": "1.0",
						"markupVersion": "1.0"
					}
				}
			},
			"apiEndpoint": "https://api.amazonalexa.com",
			"apiAccessToken": "...."
		}
	},
	"request": {
		"type": "LaunchRequest",
		"requestId": ".....",
		"timestamp": "2018-04-29T21:14:39Z",
		"locale": "en-US",
		"shouldLinkResultBeReturned": false
	}
}

so maybe we should use this flag and clear context data when think that we faced "new session".
Which actually will be "new" in dialog like:

[playing blues of 78s]
[USER] stop
[USER] play jazz

it is very likely that user means: play jazz of 78s.
but we would get new session and request of play jazz and sure forget about 78s and blues.

Ideas:

• Could state machine be useful here?
• set attributes['playbackFinished'] = true in PlaybackFinished intent