feat: replace event channel with broadcast channel

This makes `EventTracker` receive events immediately
instead of being moved from event emitter to event tracker
by a task spawned from `TestContext::new_internal`.

This makes `EventTracker.clear_events` reliable
as it is guaranteed to remove all events emitted
by the time it is called rather than only events
that have been moved already.

deltachat-rpc-server/src/main.rs

@@ -68,7 +68,7 @@ async fn main_impl() -> Result<()> {
 
     log::info!("Creating JSON-RPC API.");
     let accounts = Arc::new(RwLock::new(accounts));
-    let state = CommandApi::from_arc(accounts.clone());
+    let state = CommandApi::from_arc(accounts.clone()).await;
 
     let (client, mut out_receiver) = RpcClient::new();
     let session = RpcSession::new(client.clone(), state.clone());