Improve and fix runner API

- The server now also signals abort_work if the commit is no longer in
  the queue.
- The server now immediately reserves the work it gives to the worker,
  so other idle workers won't get the same work.
- The server now chooses a run id, not the worker. The worker can still
  submit work under arbitrary run ids when no id is known, for example
  when importing runs from another database.

src/shared.rs

@@ -108,6 +108,8 @@ pub enum BenchMethod {
 
 #[derive(Clone, Serialize, Deserialize)]
 pub struct Work {
+    /// Id of the run.
+    pub id: String,
     /// Hash of commit to benchmark.
     pub hash: String,
     /// How to benchmark the commit.