Avoid using futures' block_on in the request database guard

The request guard works by sending on a channel when it is
dropped. Since we can't do async code in `Drop::drop`, I was using
`futures::executor::block_on` under the (false!) impression that
this *specific* usecase would not be a problem as we are always
pulling from that channel. However, sending on a channel can result in
`Pending` when the Tokio worker's cooperative budget is exhausted,
even if the queue is completely empty!

A simple example:

```rust
// Drain the budget
while task::consume_budget().now_or_never().is_some() {}

// Try to use more budget than we have and block forever
futures::executor::block_on(tx.send(42u8)).unwrap();
```

This `Pending` will never resolve because we are no longer running the
Tokio runtime so it will never see an `.await` and replenish the
budget. With enough bad luck, all the worker threads get stuck like
this and the entire system becomes unavailable.

There are a number of possible workarounds, but the least invasive is
to punt the final logging off to a Tokio task and let it happen
out-of-band. Technically this will make the timing a little bit less
accurate, but I was never worried about exact nanoseconds anyway.

ui/src/request_database.rs

@@ -215,7 +215,7 @@ impl Handle {
         let g = self
             .attempt_start_request(category, payload)
             .await
-            .map(|id| EndGuardInner(id, How::Abandoned, self));
+            .map(|id| EndGuardInner(id, How::Abandoned, Some(self)));
         EndGuard(g)
     }
 }
@@ -238,12 +238,16 @@ impl EndGuard {
     }
 }
 
-struct EndGuardInner(Id, How, Handle);
+struct EndGuardInner(Id, How, Option<Handle>);
 
 impl Drop for EndGuardInner {
     fn drop(&mut self) {
-        let Self(id, how, ref handle) = *self;
-        futures::executor::block_on(handle.attempt_end_request(id, how))
+        let Self(id, how, ref mut handle) = *self;
+        if let Ok(h) = tokio::runtime::Handle::try_current() {
+            if let Some(handle) = handle.take() {
+                h.spawn(async move { handle.attempt_end_request(id, how).await });
+            }
+        }
     }
 }