rtspclientsink: Notify the stream transport about each written message

Otherwise it will never try to send us the next one: it tries to keep
exactly one message in-flight all the time.

In gst-rtsp-server this is done asynchronously via the GstRTSPWatch but
in the client sink we always write data out synchronously.
11 jobs for client-sink-send-message in 53 minutes and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #238945
gst indent

00:00:18

passed #238944
manifest

00:00:37

 
  Build
passed #238946
build fedora x86_64

00:05:21

passed #238947
build nodebug fedora x86_64

00:05:23

 
  Test
passed #238948
check fedora

00:15:49

passed #238949
integration testsuites fedora

00:27:09

 
  Full Builds
passed #238951
build cerbero android universal

00:14:04

passed #238952
build cerbero cross win32

00:07:06

passed #238953
build cerbero cross win64

00:05:01

passed #238950
build cerbero fedora x86_64

00:05:33

 
  Apps
passed #238954
android universal examples

00:06:04