Handle normalization version changes in UtteranceCache handling

[lumpidu]

Is there anywhere the possibility to findItemByTextAndFrontendVersion? If the current frontend version == cacheItem frontend version, we don't have to perform text pre-processing, isn't that correct?

Originally posted by @bnika in #102 (comment)

[lumpidu]

We should probably expire CachteItem audio, where normalized text and/or G2P has changed. Both operations are relatively quick, but nevertheless should be done in a background job and not in-line with user input.