Checkpoint SQLite WAL off the request thread safely

Disable wal_autocheckpoint and run PASSIVE checkpoints from one elected
writer process. Contenders start in console/rake (initializer), Puma
workers (including WEB_CONCURRENCY=auto), and Resque children, with
failover, capped error backoff, and tests for lock takeover.

Co-authored-by: Thomas Klemm <github@tklemm.eu>

lib/tasks/resque.rake

@@ -8,5 +8,6 @@ task "resque:pool:setup" do
   Resque::Pool.after_prefork do |job|
     ActiveRecord::Base.establish_connection
     Resque.redis.client.close
+    SqliteWalCheckpoint.start
   end
 end