Cleaning public directory is stalling startup

[tjenkinson]

If there are a lot of files in the public directory that need to be deleted when the app starts, this can take a long time and delay the amount of time before the app can function.

Instead during the initial load references to all the files that need deleting should be stored, and then the deletion should happen in a separate thread when app is ready.

[tjenkinson]

The issue: https://github.com/LA1TV/DVR-Bridge-Service/blob/cd55c10/src/main/uk/co/la1tv/dvrBridgeService/servableFiles/ServableFileGenerator.java#L58