Error polling Segment: SegmenterTimeout when RTMP content input with frame rates <=25

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### Summary

When streaming RTMP content into livepeer in broadcaster mode, the streams fail when the content is <= 25 frames per second.

The failure is reported as a `SegmenterTimeout`.

Streams of RTMP content with >25 FPS are processed successfully.

### More Detail

When using version 0.4.0, started with `./livepeer -offchain -httpAddr 0.0.0.0:8935 -rtmpAddr 0.0.0.0:1935 -currentManifest`

`SegmenterTimeout` errors when broadcasting from OBS at FPS of presets `24 NTSC`, `25 PAL`, `20`, `10`, and manually set `25`, `24`, `15`, `1`.

**NO** `SegmenterTimeout` errors when broadcasting from OBS at FPS of presets `60`, `59.94`, `50 PAL`, `48`, `30`, `29.97` and manually set: `29`, `28`, `27`, `26`

### Error Log

```
E0705 14:31:02.641940 2867 lpms.go:172] Error polling Segment: SegmenterTimeout, Retrying
E0705 14:31:03.142456 2867 lpms.go:172] Error polling Segment: SegmenterTimeout, Retrying
E0705 14:31:03.642891 2867 lpms.go:198] Error segmenting stream: SegmenterTimeout
E0705 14:31:03.643307 2867 basic_rtmp_videostream.go:109] Error reading packet from RTMP: read tcp 178.62.235.51:1935->2.4.142.235:55648: use of closed network connection
```

Reproduce the issue with the provided livepeer command and OBS at frame rates of 25 FPS or lower. Start by inspecting the SegmenterTimeout handling at lpms.go:172 and lpms.go:198, then trace the RTMP read path in basic_rtmp_videostream.go:109. Done means low-frame-rate RTMP streams no longer fail with SegmenterTimeout.

Written by the indexing model from the issue text.