rtpjitterbuffer: make sure not to drop packets based on skew

One of the jitterbuffers functions is to try and make sense of weird
network behavior.

It is quite unhelpful for the jitterbuffer to start dropping packets
itself when what you are trying to achieve is better network resilience.

In the case of a skew, this could often mean the sender has restarted
in some fashion, and then dropping the very first buffer of this "new"
stream could often mean missing valuable information, like in the case
of video and I-frames.

This patch simply reverts back to the old behavior, prior to gstreamer/gst-plugins-good@8d955fc3
and includes the simplest test I could write to demonstrate the behavior,
where a single packet arrives "perfectly", then a 50ms gap happens,
and then two more packets arrive in perfect order after that.

# Conflicts:
#	tests/check/elements/rtpjitterbuffer.c
25 jobs for jitterbuffer-dont-drop-skew-packets in 54 minutes and 8 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #779137
gst indent

00:00:36

passed #779136
manifest

00:00:43

 
  Build
passed #779147
build cerbero cross-android universal

00:19:01

manual #779151
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #779148
build cerbero cross win32

00:11:59

passed #779149
build cerbero cross win64

00:08:00

passed #779146
build cerbero fedora x86_64

00:06:50

passed #779150
gst-macos-10.14
build cerbero macos x86_64

00:06:48

passed #779142
build clang fedora x86_64

00:09:08

passed #779138
build fedora x86_64

00:08:11

passed #779145
docker windows 1607
build msys2

00:11:19

passed #779139
build nodebug fedora x86_64

00:05:22

passed #779140
build static fedora x86_64

00:14:54

passed #779141
build static nodebug fedora x86_64

00:05:47

passed #779143
docker windows 1607
build vs2017 amd64

00:09:38

passed #779144
docker windows 1607
build vs2017 x86

00:09:36

 
  Test
passed #780212
check fedora

00:04:26

passed #779153
integration testsuites fedora

00:40:43

passed #779155
valgrind bad

00:09:24

passed #779156
valgrind ges

00:09:59

passed #779154
valgrind good

00:11:36

failed #779152
check fedora

00:05:41

 
  Integrate
passed #779157
cross-android universal examples

00:08:05

manual #779159
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #779158
documentation

00:03:55