rtpbin: add request-jitterbuffer signal

This can be used to pass the threadsharing jitterbuffer from
gst-plugins-rs for example.
26 jobs for rtpbin-request-jb in 56 minutes and 54 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #590496
gst indent

00:00:43

passed #590495
manifest

00:01:08

 
  Build
passed #591128
build cerbero android universal

00:17:06

passed #590507
build cerbero cross win32

00:10:09

passed #590508
build cerbero cross win64

00:07:26

passed #590505
build cerbero fedora x86_64

00:07:33

manual #590510
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #590509
gst-macos-10.14
build cerbero macos x86_64

00:07:24

passed #590501
build clang fedora x86_64

00:07:12

passed #590497
build fedora x86_64

00:04:16

passed #590504
docker windows 1607
build msys2

00:10:58

passed #590498
build nodebug fedora x86_64

00:03:54

passed #590499
build static fedora x86_64

00:12:52

passed #590500
build static nodebug fedora x86_64

00:03:33

passed #590502
docker windows 1607
build vs2017 amd64

00:07:59

passed #590503
docker windows 1607
build vs2017 x86

00:09:27

failed #590506
build cerbero android universal

01:00:02

 
  Test
passed #590511
check fedora

00:07:34

passed #590512
integration testsuites fedora

00:25:49

passed #590514
valgrind bad

00:16:16

passed #590515
valgrind ges

00:06:54

passed #590513
valgrind good

00:21:38

 
  Integrate
passed #590516
android universal examples

00:08:32

passed #591129
documentation

00:05:59

manual #590518
gst-ios-12.2 allowed to fail manual
ios universal examples
failed #590517
documentation

01:56:19