Fix the timer that periodically saves the seektable delta.

After 309c97e3001b7b2748c42564d5b93b923d00a171 and related commits,
the positionMapTimer never actually got started, and seektable updates
weren't written out until the recording ended.  This affected seeking
in live TV and in-progress recordings, and because the file size
wasn't being updated either, it actually prevented starting playback
of in-progress recordings.