Randomize temporary ogg file

[Daniel-Mietchen]

Currently, the temporary ogg file is always named "current.ogg", which causes problems in case several instances are running. Better to replace with random file names and to clean up when done with upload.

[RaphaelWimmer]

Concretely, use Python's tempfile.mkstemp().

Note: This does not avoid two simultaneously running bots overwriting each others final output files - it just avoids this problem during transcoding.