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.
19 jobs for jitterbuffer-big-gap in 47 minutes and 55 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Preparation
passed #403790
gst indent

00:00:33

passed #403789
manifest

00:00:45

 
  Build
passed #403796
build cerbero android universal

00:12:56

passed #403797
build cerbero cross win32

00:11:12

passed #403798
build cerbero cross win64

00:08:26

passed #403795
build cerbero fedora x86_64

00:07:21

manual #403800
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #403799
gst-macos-10.14
build cerbero macos x86_64

00:06:42

passed #403791
build fedora x86_64

00:10:17

passed #403792
build nodebug fedora x86_64

00:08:14

passed #403793
build static fedora x86_64

00:16:36

passed #403794
build static nodebug fedora x86_64

00:16:19

 
  Test
failed #403801
check fedora

00:05:36

passed #403802
integration testsuites fedora

00:25:09

passed #403804
valgrind bad

00:14:23

passed #403805
valgrind ges

00:09:28

passed #403803
valgrind good

00:18:58

 
  Integrate
skipped #403806
android universal examples
skipped #403807
documentation
 
Name Stage Failure
failed
check fedora Test
           Total: 3099
Uploading artifacts...
gst-build/build/meson-logs/: found 2 matching files
validate-logs: found 3517 matching files

Uploading artifacts to coordinator... ok
id=403801 responseStatus=201 Created token=xqt6XLz8
Uploading artifacts...
validate-logs/*.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=403801 responseStatus=201 Created token=xqt6XLz8
ERROR: Job failed: exit code 1