Refactor stat_collector to run all collections in async parallel

[cooperlees]

• Now that we have an async dbus client, lets make this function
• Add an update_*_stats API in each module + pass a RWLocked stats struct to update
• Join and check for any errors + log etc.

Test Plan:

• Run and see same stats
• Didn't notice much runtime win here ... down from 12-13ms to 11-12ms
  • 10ms if I disable networkd
  • Still got some networkd I/O to make async ... will do that in a follow up dedicated PR

    
    cooper@cooper-fedora-MJ0J8MTZ:~/repos/monitord$ ls -lh target/release/monitord
    -rwxr-xr-x. 2 cooper cooper 3.5M Oct  1 09:04 target/release/monitord

cooper@cooper-fedora-MJ0J8MTZ:~/repos/monitord$ time ./target/release/monitord -c monitord.conf > /dev/null I1001 09:05:16.258015 3289269 src/main.rs:28] monitord: Know how happy your systemd is! 😊 I1001 09:05:16.258750 3289269 src/lib.rs:76] Starting stat collection run E1001 09:05:16.259904 3289269 src/networkd.rs:308] Unable to get interface links via DBUS - is networkd running?: MethodError( OwnedErrorName( "org.freedesktop.DBus.Error.NameHasNoOwner", ), Some( "Could not activate remote peer 'org.freedesktop.network1': activation request failed: unknown unit", ), Msg { type: Error, sender: UniqueName( "org.freedesktop.DBus", ), reply-serial: 2, body: Signature( "s", ), fds: [], }, ) I1001 09:05:16.269851 3289269 src/lib.rs:142] stat collection run took 11ms

real 0m0.020s user 0m0.005s sys 0m0.026s

- No networkd running on Fedora