random_init_delay: remove the minimum of 10 seconds (#3914)

Before this patch, the range from which the random delay is picked is at
minimum 10 seconds.
With this patch, they delay is bounded to whatever the given `period`
is, and zero, if period id Duration::ZERO.

Motivation for this: the disk usage eviction tests that we'll add in
https://github.com/neondatabase/neon/pull/3905 need to wait for the disk
usage eviction background loop to do its job.
They set a period of 1s.
It seems wasteful to wait 10 seconds in the tests.

Co-authored-by: Joonas Koivunen <joonas@neon.tech>

pageserver/src/tenant/tasks.rs

@@ -244,14 +244,12 @@ pub(crate) async fn random_init_delay(
 ) -> Result<(), Cancelled> {
     use rand::Rng;
 
+    if period == Duration::ZERO {
+        return Ok(());
+    }
+
     let d = {
         let mut rng = rand::thread_rng();
-
-        // gen_range asserts that the range cannot be empty, which it could be because period can
-        // be set to zero to disable gc or compaction, so lets set it to be at least 10s.
-        let period = std::cmp::max(period, Duration::from_secs(10));
-
-        // semi-ok default as the source of jitter
         rng.gen_range(Duration::ZERO..=period)
     };