rtpjitterbuffer: improve calculation of jitter and add more stats

Given a constant source of jitter, the total-avg-jitter will be the
"correct" amount of jitter experienced.

Comparing this number to the running average for many long-running tests,
having a slightly larger window seems to approximate the jitter much
better.

In addition we avoid weighting 0 too much when the initial calculations
are done by ramping up the window and not counting the very first
measurement.
24 jobs for jitterbuffer-big-gap in 121 minutes and 58 seconds (queued for 6 seconds)
latest
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 win32

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build cerbero cross win64

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build cerbero fedora x86_64

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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
build vs2017 amd64

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docker windows 1607
build vs2017 x86

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  Test
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check fedora

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integration testsuites fedora

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valgrind bad

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  Integrate
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cross-android universal examples

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cross-ios universal examples
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documentation

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