agent: Replace closed signal with GASyncResult type function

This makes the API more GIO style
6 jobs for async-turn-remove-allocation in 2 minutes and 52 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #289471
build autotools

00:00:52

passed #289472
build meson

00:00:25

 
  Test
passed #289474
distcheck autotools

00:01:59

passed #289476
doc-and-install meson

00:00:45

passed #289473
test autotools

00:01:21

passed #289475
allowed to fail
test meson

00:00:47