kvssink hard start/stop support

[stefankiesz]

Tested using: kvssink sample to start/stop the stream at various intervals including:

• ~100 times in a run with 0s interval between start/stops
• play 5s -> sleep 5s -> play 20s -> sleep 60s -> play 20s -> sleep 20min -> play onward

The stream was inspected for successful ingestion at each playing stage.

Long-term memory consumption still needs to be verified.

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[codecov-commenter]

Codecov Report

Attention: 237 lines in your changes are missing coverage. Please review.

Comparison is base (6af9a7a) 16.38% compared to head (e78ea19) 16.59%. Report is 4 commits behind head on develop.

:exclamation: Current head e78ea19 differs from pull request most recent head 3e63221. Consider uploading reports for the commit 3e63221 to get more accurate results

Files	Patch %	Lines
samples/kvssink_gstreamer_sample.cpp	0.00%	106 Missing :warning:
samples/gstreamer_sandbox.cpp	0.00%	66 Missing :warning:
src/gstreamer/gstkvssink.cpp	0.00%	56 Missing :warning:
samples/kvs_gstreamer_audio_video_sample.cpp	0.00%	9 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1120      +/-   ##
===========================================
+ Coverage    16.38%   16.59%   +0.21%     
===========================================
  Files           51       51              
  Lines         6849     6973     +124     
===========================================
+ Hits          1122     1157      +35     
- Misses        5727     5816      +89     

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