Add SystemClock (#8239)

This PR adds a `SystemClock` trait for abstracting away the system
clock.

This allows us to swap out the real system clock with a
`FakeSystemClock` in the tests, thus allowing the fake passage of time.

We're using this in `Telemetry` to better mock the clock for testing
purposes.

Release Notes:

- N/A

crates/channel/src/channel_store_tests.rs

@@ -2,6 +2,7 @@ use crate::channel_chat::ChannelChatEvent;
 
 use super::*;
 use client::{test::FakeServer, Client, UserStore};
+use clock::FakeSystemClock;
 use gpui::{AppContext, Context, Model, TestAppContext};
 use rpc::proto::{self};
 use settings::SettingsStore;
@@ -337,8 +338,9 @@ fn init_test(cx: &mut AppContext) -> Model<ChannelStore> {
     release_channel::init("0.0.0", cx);
     client::init_settings(cx);
 
+    let clock = Arc::new(FakeSystemClock::default());
     let http = FakeHttpClient::with_404_response();
-    let client = Client::new(http.clone(), cx);
+    let client = Client::new(clock, http.clone(), cx);
     let user_store = cx.new_model(|cx| UserStore::new(client.clone(), cx));
 
     client::init(&client, cx);