Do not start stream twice if `encode.ffmpeg_stream_restart` is true

[AlexanderS]

Is your feature request related to a problem? Please describe.
If I have encode.ffmpeg_stream_restart enabled and the box wants to continue a stream, an ffmpeg process starts twice, and the first one quits just after the buffer period.

Describe the solution you'd like
f I understand correctly, encode.ffmpeg_stream_restart works by closing the file at the location where the box tries to continue the stream. Would it be possible to close the file immediately on request without starting the unnecessary ffmpeg process? This would considerably speed up the startup of the actual stream.

Describe alternatives you've considered
If I disable encode.ffmpeg_stream_restart and use a web radio stream, the box continues to play the cached data and suddenly jumps to more recent content.