Stop WAL checkpointer before fork; one thread owns the flock

Unify startup: initializer starts every non-test process, Puma and Resque
stop before fork and start again in the child. Only the contender thread
releases the lock. Drop Puma::CLI / single_puma_process? special cases.

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

lib/tasks/resque.rake

@@ -4,6 +4,9 @@ end
 
 task "resque:pool:setup" do
   ActiveRecord::Base.connection.disconnect!
+  # Initializer may have started a contender in the pool master; drop it before
+  # workers fork so they do not inherit the flock.
+  SqliteWalCheckpoint.stop
 
   Resque::Pool.after_prefork do |job|
     ActiveRecord::Base.establish_connection