core: when reloading, delay any actions on journal and dbus connections

core: when reloading, delay any actions on journal and dbus connections

manager_recheck_journal() and manager_recheck_dbus() would be called to early
while we were deserialiazing units, before the systemd-journald.service and
dbus.service have been deserialized. In effect we'd disable logging to the
journald and close the bus connection. The first is not very noticable, it
mostly means that logs emitted during deserialization are lost. The second is
more noticeable, because manager_recheck_dbus() would call bus_done_api() and
bus_done_system() and close dbus connections. Logging and bus connection would
then be restored later after the respective units have been deserialized.

This is easily reproduced by calling:
  $ sudo gdbus call --system --dest org.freedesktop.systemd1 --object-path /org/freedesktop/systemd1 --method "org.freedesktop.systemd1.Manager.Reload"
which works fine before 8559b3b75cb, and then starts failing with:
  Error: GDBus.Error:org.freedesktop.DBus.Error.NoReply: Remote peer disconnected

None of this should happen, and we should delay changing state until after
deserialization is complete when reloading. manager_reload() already included
the calls to manager_recheck_journal() and manager_recheck_dbus(), so the
connection state will be updated after deserialization during reloading is done.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1554578.