firewall: refactor "nm-firewall-manager.c" to not use GDBusProxy

- Don't use GDBusProxy but plain GDBusConnection. NMFirewallManager
  is very simple, it doesn't use any of the features that GDBusProxy
  provides.

- make NMFirewallManagerCallId typedef a pointer to the opaque call-id
  struct, instead of the struct itself. It's confusing to have a
  variable that does not look like a pointer and assigning %NULL to
  it.

- internally drop the CBInfo typename and name the call-id variable
  constsistantly as "call_id".

- no need to keep the call-id struct alive after cancelling it. That
  simplifies the lifetime managment of the pending call because the
  completion callback is always invoked shortly before destroying
  the call-id.

- note that the caller is no longer allowed to cancel a call-id from
  inside the completion callback. That just complicates the
  implementation and is not necessary. Assert against that.
15 jobs for th/firewall-manager-rework in 43 minutes and 34 seconds (queued for 407 minutes and 53 seconds)
Status Job ID Name Coverage
  Test
passed #476827
allowed to fail
checkpatch

00:01:32

manual #476832
allowed to fail manual
t_centos:7.5.1804
passed #476833
t_centos:7.6.1810

00:42:01

passed #476839
t_debian:10

00:18:51

passed #476838
t_debian:9

00:17:30

passed #476842
t_debian:sid

00:16:13

manual #476840
allowed to fail manual
t_debian:testing
passed #476828
t_fedora:28

00:28:29

passed #476829
t_fedora:29

00:33:35

passed #476830
t_fedora:30

00:26:41

manual #476831
allowed to fail manual
t_fedora:rawhide
passed #476834
t_ubuntu:16.04

00:19:46

passed #476835
t_ubuntu:18.04

00:33:37

manual #476837
allowed to fail manual
t_ubuntu:devel
manual #476836
allowed to fail manual
t_ubuntu:rolling