fix: Use SystemTime instead of Instant everywhere

If a time value doesn't need to be sent to another host, saved to the db or otherwise used across
program restarts, a monotonically nondecreasing clock (`Instant`) should be used. But as `Instant`
may use `libc::clock_gettime(CLOCK_MONOTONIC)`, e.g. on Android, and does not advance while being in
deep sleep mode, get rid of `Instant` in favor of using `SystemTime`, but add `tools::Time` as an
alias for it with the appropriate comment so that it's clear why `Instant` isn't used in those
places and to protect from unwanted usages of `Instant` in the future. Also this can help to switch
to another clock impl if we find any.

src/tools.rs

@@ -9,6 +9,13 @@ use std::io::{Cursor, Write};
 use std::mem;
 use std::path::{Path, PathBuf};
 use std::str::from_utf8;
+// If a time value doesn't need to be sent to another host, saved to the db or otherwise used across
+// program restarts, a monotonically nondecreasing clock (`Instant`) should be used. But as
+// `Instant` may use `libc::clock_gettime(CLOCK_MONOTONIC)`, e.g. on Android, and does not advance
+// while being in deep sleep mode, we use `SystemTime` instead, but add an alias for it to document
+// why `Instant` isn't used in those places. Also this can help to switch to another clock impl if
+// we find any.
+pub use std::time::SystemTime as Time;
 use std::time::{Duration, SystemTime};
 
 use anyhow::{bail, Context as _, Result};
@@ -482,6 +489,10 @@ pub(crate) fn time() -> i64 {
         .as_secs() as i64
 }
 
+pub(crate) fn time_elapsed(time: &Time) -> Duration {
+    time.elapsed().unwrap_or_default()
+}
+
 /// Struct containing all mailto information
 #[derive(Debug, Default, Eq, PartialEq)]
 pub struct MailTo {