Construct event channel outside of Context

This allows account manager to construct a single event channel and
inject it into all created contexts instead of aggregating events from
separate event emitters.

benches/contacts.rs

@@ -2,13 +2,16 @@ use async_std::task::block_on;
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
 use deltachat::contact::Contact;
 use deltachat::context::Context;
+use deltachat::Events;
 use tempfile::tempdir;
 
 async fn address_book_benchmark(n: u32, read_count: u32) {
     let dir = tempdir().unwrap();
     let dbfile = dir.path().join("db.sqlite");
     let id = 100;
-    let context = Context::new(dbfile.into(), id).await.unwrap();
+    let context = Context::new(dbfile.into(), id, Events::new())
+        .await
+        .unwrap();
 
     let book = (0..n)
         .map(|i| format!("Name {}\naddr{}@example.org\n", i, i))