Move to varlink if/when APIs are available

[cooperlees]

There is world that the dbus calls we use are possibly moving to be exposed via varlink on PID 1 itself. If that happens lets use it as there are less dependencies and cheaper lookups etc. etc.

• The first few versions of monitord will support both and config will drive which is used
• Eventually we'll remove dbus support once a version of systemd with varlink is mainstream

Experiment using async varlink client:

• https://github.com/zeenix/zlink

[cooperlees]

Looking today at systemd APIs, still seems we need a lot more implemented before monitord can go anywhere near varlink

• There is also still no async/tokio client

[root@b89a51f4c266 systemd]# find . | grep 'io\.'
./userdb/io.systemd.DynamicUser
./userdb/io.systemd.Multiplexer
./userdb/io.systemd.NameServiceSwitch
./userdb/io.systemd.DropIn
./userdb/io.systemd.Home
./io.systemd.ManagedOOM
./io.systemd.Credentials
./journal/io.systemd.journal
./oom/io.systemd.ManagedOOM
./io.systemd.BootControl
./io.systemd.sysext
./netif/io.systemd.Network
./resolve/io.systemd.Resolve
./resolve/io.systemd.Resolve.Monitor
./io.systemd.Hostname

• This is with a pretty new systemd too (257.3-7.fc43)

Going to park it for awhile.

[cooperlees]

systemd/systemd#36755 - Posted asking for more documentation and a way to see when the APIs we need are available etc. etc.

[cooperlees]

systemd/systemd#36877 - Asking for new varlink API to move off parsing the private networkd files we parse ...

[cooperlees]

Also, want to watch / move to systemd/systemd#38023 once we can.

[cooperlees]

#158 starts this move for main unit stats for monitord.