Backport/playback queue singleton

[forslund]

Description

This backports the TTS Playback queue singleton PR to the dev branch.

This fixes some additional issues (existing on dev at least)

• Handle Mimic2 error on chunk without repetition
• some minor changes where self was used instead of TTS to refer to the singleton for consistency
• Curate cache for all attached TTS's
• Remove use of clear_cache in Mimic (1) TTS implementation, this caused all cache to be removed from Mimic2 when fallback tts was called.

How to test

The game-zork skill triggers really bad issues when using Mimic2, (500 errors on almost empty utterances) and together with the listening=true it's really noticable. Playing that for a bit should give some idea if it works or not.

Contributor license agreement signed?

CLA [x]

[devops-mycroft]

Voight Kampff Integration Test Succeeded (Results)