rtpjitterbuffer: Cancel timers instead of just unlocking loop thread

When the queue is full (and adding more packets would risk a seqnum
roll-over), the best approach is to just start pushing out packets
from the other side.  Just pushing out the packets results in the
timers being left hanging with old seqnums, so it's safer to just
execute them immediately in this case. It does limit the timer space
to the time it takes to receiver about 32k packets, but without
extended sequence number, this is the best RTP can do.

This also results in the test no longer needed to have timeouts or
timers as pushing packets in drives everything.

Fixes #619
26 jobs for 619-fix-flaky-jitterbuffer-queue-filling-test in 56 minutes and 30 seconds (queued for 2 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 cerbero cross-android universal

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build cerbero cross-ios universal
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build cerbero cross win32

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build cerbero cross win64

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gst-macos-10.14
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build clang fedora x86_64

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build fedora x86_64

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docker windows 1607
build msys2

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build nodebug fedora x86_64

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build static fedora x86_64

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build static nodebug fedora x86_64

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docker windows 1607
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docker windows 1607
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  Test
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check fedora

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integration testsuites fedora

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  Integrate
passed #674437
cross-android universal examples

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manual #674439
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #674438
documentation

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