splitmuxsink: send flush_start event to the muxer first

For aggregator-based muxers, it allows aggregator to handle
flushing in a threadsafe manner

See gstreamer/gstreamer!159
16 jobs for splitmuxsink-flush-muxer in 32 minutes and 33 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
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gst indent

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manifest

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  Build
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build fedora x86_64

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build nodebug fedora x86_64

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  Test
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check fedora

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integration testsuites fedora

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  Full Builds
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build cerbero android universal

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build cerbero cross win32

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build cerbero cross win64

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passed #326468
build cerbero fedora x86_64

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  Full Tests
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valgrind bad

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valgrind ges

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failed #348545
valgrind good

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valgrind good

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  Apps
skipped #326475
android universal examples
 
  Deploy
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documentation
 
Name Stage Failure
failed
valgrind good Full Tests There has been a missing dependency failure
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