Restructure shared types and db columns

Now, the server sends the runner pretty much all run metadata. This way,
the reservation the server makes for the runner is accurate, providing
the runner responds with the same metadata it was sent. It also means
that only the server's system clock is relevant for tie breakers, and a
run's duration spans from the moment it is reserved to the moment it is
saved.

Also, the bench method is now always called `bench_method` and a
human-readable description is stored in the database for each run.

migrations/20230809141314_runs.sql

@@ -1,9 +1,10 @@
 CREATE TABLE runs (
-    id        TEXT NOT NULL PRIMARY KEY,
-    hash      TEXT NOT NULL,
-    start     TEXT NOT NULL,
-    end       TEXT NOT NULL,
-    exit_code INT  NOT NULL,
+    id           TEXT NOT NULL PRIMARY KEY,
+    hash         TEXT NOT NULL,
+    bench_method TEXT NOT NULL,
+    start        TEXT NOT NULL,
+    end          TEXT NOT NULL,
+    exit_code    INT  NOT NULL,
 
     FOREIGN KEY (hash) REFERENCES commits (hash) ON DELETE CASCADE
 ) STRICT;