Rename zed-server to collab

Over time, I think we may end up having multiple services, so it seems like a good opportunity to name this one more specifically while the cost is low. It just seems like naming it "zed" and "zed-server" leaves it a bit open ended.

crates/collab/src/expiring.rs

@@ -0,0 +1,43 @@
+use std::{future::Future, time::Instant};
+
+use async_std::sync::Mutex;
+
+#[derive(Default)]
+pub struct Expiring<T>(Mutex<Option<ExpiringState<T>>>);
+
+pub struct ExpiringState<T> {
+    value: T,
+    expires_at: Instant,
+}
+
+impl<T: Clone> Expiring<T> {
+    pub async fn get_or_refresh<F, G>(&self, f: F) -> tide::Result<T>
+    where
+        F: FnOnce() -> G,
+        G: Future<Output = tide::Result<(T, Instant)>>,
+    {
+        let mut state = self.0.lock().await;
+
+        if let Some(state) = state.as_mut() {
+            if Instant::now() >= state.expires_at {
+                let (value, expires_at) = f().await?;
+                state.value = value.clone();
+                state.expires_at = expires_at;
+                Ok(value)
+            } else {
+                Ok(state.value.clone())
+            }
+        } else {
+            let (value, expires_at) = f().await?;
+            *state = Some(ExpiringState {
+                value: value.clone(),
+                expires_at,
+            });
+            Ok(value)
+        }
+    }
+
+    pub async fn clear(&self) {
+        self.0.lock().await.take();
+    }
+}