Port GHashTable to gatomicrefcount

Use the newly added API for reference counting instead of rolling our
own.
6 jobs for refcount-types in 20 minutes and 23 seconds (queued for 11 seconds)
Status Job ID Name Coverage
  Build
passed #45993
fedora-meson-android_ndk_r16_api21_arm64

00:06:30

passed #45994
fedora-meson-mingw64

00:06:12

passed #45992
fedora-meson-x86_64

00:08:23

passed #45995
win32
msys2-mingw32

00:05:37

passed #45996
win32
vs2017-x64

00:05:38

 
  Coverage
passed #45997
coverage

00:05:47

76.9%