systemd: use LogsDirecoty= to persist data

Previously used `StateDirectory=` to persist pinger specific
data across bootups. It turns out that systemd does not allow
sharing state directory across different dynamic users. An example
would be:

```
$ systemd-run --pty --property=DynamicUser=yes --property=StateDirectory=wuff /bin/sh
$ systemd-run --pty --property=DynamicUser=yes --property=StateDirectory=wuff /bin/sh
```

Reference: http://0pointer.net/blog/dynamic-users-with-systemd.html
Signed-off-by: Allen Bai <abai@redhat.com>

dist/systemd/fedora-coreos-pinger.service

@@ -9,6 +9,7 @@ After=network-online.target
 DynamicUser=yes
 Type=oneshot
 RemainAfterExit=yes
+LogsDirectory=fedora-coreos-pinger
 ExecStart=/usr/libexec/fedora-coreos-pinger
 
 [Install]

src/main.rs

@@ -114,7 +114,7 @@ fn main() -> Fallible<()> {
 
         // spawn thread for monitoring timestamp and sending report daily
         let daily_thread = thread::spawn(move || -> Fallible<()> {
-            const DAILY_TIMESTAMP_FILE: &str = r#"/var/lib/fedora-coreos-pinger/timestamp_daily"#;
+            const DAILY_TIMESTAMP_FILE: &str = r#"/var/log/fedora-coreos-pinger/timestamp_daily"#;
             const SECS_PER_12_HOURS: Duration = Duration::from_secs(12 * 60 * 60);
             loop {
                 let clock = util::Clock::read_timestamp(DAILY_TIMESTAMP_FILE)?;
@@ -133,7 +133,7 @@ fn main() -> Fallible<()> {
         // spawn thread for monitoring timestamp and sending report monthly
         let monthly_thread = thread::spawn(move || -> Fallible<()> {
             const MONTHLY_TIMESTAMP_FILE: &str =
-                r#"/var/lib/fedora-coreos-pinger/timestamp_monthly"#;
+                r#"/var/log/fedora-coreos-pinger/timestamp_monthly"#;
             const SECS_PER_15_DAYS: Duration = Duration::from_secs(15 * 24 * 60 * 60);
             loop {
                 let clock = util::Clock::read_timestamp(MONTHLY_TIMESTAMP_FILE)?;