Cache concatenated audio

[scy]

As 46elks refuses to accept streaming Vorbis files, the (upcoming) IVR code ensures that concatenated IVR files will not be deleted automatically. However, since they’re essentially temporary files as far as the concatenation code is concerned, they have random, non-deterministic names and a new one will be created on each request, even if it’s for the same medialist ID. This means

• unnecessary work to concatenate the files when a cached copy could be used
• and (worse): they will clog up the disk at some point.

Short-term solution is to remove these files every now and then.

Long-term solution is to make sure that these files use the medialist ID in their name so that they won’t be recreated for every request.

Make sure that you don’t run into race conditions when two clients request the same ID at the same time.