fixes #59

Pretty sure this is the cause of the flakiness because if you set:

diff --git a/src/server/segcache/tests/common.rs b/src/server/segcache/tests/common.rs
index 9f33681..e551a35 100644
--- a/src/server/segcache/tests/common.rs
+++ b/src/server/segcache/tests/common.rs
@@ -235,7 +235,7 @@ fn test(name: &str, data: &[(&str, Option<&str>)]) {
             }
         }

-        std::thread::sleep(Duration::from_millis(10));
+        std::thread::sleep(Duration::from_millis(1000));
         let mut buf = vec![0; 4096];

         if let Some(response) = response {

so you enforce a long wait in between running commands, then the test will fail every time on my machine (x86_64 Ubuntu WSL2).

I think the reason it doesn't fail all the time is due to machine load/architecture performance differences that cause the GET to occur either before or after the relatively immediate expiration of the key. If Pelikan was slower we'd probably have found this bug already.

The test that specifically fails (or at least the one that seems to be the cause of the nondeterministic failures) is here:

If I run that sequence of commands on memcached, the get command will always succeed, regardless of how much time has passed.

However, when I run those commands against segcache, the key is expired by the time I try to get it (at least that's the likely behavior I see, based on the GET returning nothing)