aggregator: provide a way to flush without seeking

See gstreamer/gst-plugins-bad#977
18 jobs for aggregator-flush-api in 57 minutes and 41 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #313024
gst indent

00:00:15

passed #313023
manifest

00:00:51

 
  Build
passed #313025
build fedora x86_64

00:06:30

passed #313026
build nodebug fedora x86_64

00:03:50

 
  Test
passed #313027
check fedora

00:04:54

passed #313028
integration testsuites fedora

00:09:14

 
  Full Builds
passed #313030
build cerbero android universal

00:18:04

passed #313031
build cerbero cross win32

00:12:14

passed #313032
build cerbero cross win64

00:09:56

passed #313029
build cerbero fedora x86_64

00:10:50

 
  Full Tests
passed #313037
valgrind bad

00:04:32

passed #313034
valgrind base

00:11:06

passed #313033
valgrind core

00:09:21

passed #313038
valgrind ges

00:07:59

passed #313035
valgrind good

00:12:17

passed #313036
valgrind ugly

00:01:16

 
  Apps
passed #313039
android universal examples

00:07:06

 
  Deploy
passed #313040
documentation

00:03:35