aggregator: Correct locking around segment fields

`segment` is protected by the pad's object lock. `head_segment` is
protected by the pad lock.

Simplify the use of `update_time_level` to require the pad lock be held
and the pad's object lock be free. It's called in various contexts where
it needs the object lock, or it may not.
35 jobs for aggregator-locking in 1 minute and 24 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build Docker
passed #4586480
alpine amd64 manifest builder docker

00:00:33

passed #4586479
android docker

00:00:34

passed #4586478
cerbero fedora amd64 docker

00:00:34

passed #4586477
fedora amd64 docker

00:00:34

passed #4586481
gst-indent amd64 docker

00:00:43

 
  Preparation
passed #4586483
gst indent

00:00:35

manual #4586482
manual
manifest
 
  Build
created #4586492
gstreamer
build cerbero cross-android universal
created #4586496
gst-ios-13.2
build cerbero cross-ios universal
created #4586493
gstreamer
build cerbero cross win32
created #4586494
gstreamer
build cerbero cross win64
created #4586491
gstreamer
build cerbero fedora x86_64
created #4586495
gst-macos-10.15
build cerbero macos x86_64
created #4586497
1809 gstreamer-windows windows docker allowed to fail manual
build cerbero msvc x86_64
created #4586487
build clang fedora x86_64
created #4586490
1809 windows docker allowed to fail manual
build msys2
created #4586484
build nodebug fedora x86_64
created #4586485
build static fedora x86_64
created #4586486
build static nodebug fedora x86_64
created #4586488
1809 windows docker
build vs2017 amd64
created #4586489
1809 windows docker
build vs2017 x86
 
  Test
created #4586498
gstreamer
check fedora
created #4586499
gstreamer
integration testsuites fedora 1/4
created #4586500
gstreamer
integration testsuites fedora 2/4
created #4586501
gstreamer
integration testsuites fedora 3/4
created #4586502
gstreamer
integration testsuites fedora 4/4
created #4586507
gstreamer
valgrind bad
created #4586504
gstreamer
valgrind base
created #4586503
gstreamer
valgrind core
created #4586508
gstreamer
valgrind ges
created #4586505
gstreamer
valgrind good
created #4586506
gstreamer
valgrind ugly
 
  Integrate
created #4586509
cross-android universal examples
created #4586511
gst-ios-13.2
cross-ios universal examples
created #4586510
documentation