Exclusively lock Runners during code executions

Previously, the same runner could be used multiple times with different submissions simultaneously. This, however, yielded errors, for example when one submission time oud (causing the running to be deleted) while another submission was still executed.

Admin actions, such as the shell, can be still executed regardless of any other code execution.

Fixes CODEOCEAN-HG
Fixes openHPI/poseidon#423

db/migrate/20231029172331_add_reserved_until_to_runners.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddReservedUntilToRunners < ActiveRecord::Migration[7.1]
+  def change
+    add_column :runners, :reserved_until, :datetime, null: true, default: nil
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2023_10_13_194635) do
+ActiveRecord::Schema[7.1].define(version: 2023_10_29_172331) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
   enable_extension "pgcrypto"
@@ -483,6 +483,7 @@ ActiveRecord::Schema[7.1].define(version: 2023_10_13_194635) do
     t.bigint "contributor_id"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.datetime "reserved_until"
     t.index ["contributor_type", "contributor_id"], name: "index_runners_on_user"
     t.index ["execution_environment_id"], name: "index_runners_on_execution_environment_id"
   end