pad: clear sticky event tag upon stream-start

When playing gapless there were situations when some sticky events
like tags were stuck at some pad and then revived much later.
Therefore it is better to clear them upon stream-start.

Fixes #360
9 jobs for NTO-pad-tag-fix in 33 minutes and 14 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #133738
gst indent

00:00:15

passed #133737
manifest

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  Build
passed #133739
build fedora x86_64

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  Test
passed #133740
check fedora

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  Full Builds
passed #133741
build android arm64 api28

00:02:53

passed #133743
build cerbero cross win32

00:09:06

passed #133744
build cerbero cross win64

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passed #133761
build cerbero fedora x86_64

00:06:53

failed #133742
build cerbero fedora x86_64

00:02:52