Fix hang on shutdown in test_refork [changelog skip]

[wjordan]

Description

This PR fixes a hang on shutdown that occurred intermittently in test_refork (example). I reproduced the hang more consistently running test_refork with an extra sleep after the 'b' (boot) message and before it starts the server (e.g., line 106 below):

puma/lib/puma/cluster/worker.rb

Lines 99 to 108 in aae4d4d

	begin
	@worker_write << "b#{Process.pid}:#{index}\n"
	rescue SystemCallError, IOError
	Thread.current.purge_interrupt_queue if Thread.current.respond_to? :purge_interrupt_queue
	STDERR.puts "Master seems to have exited, exiting."
	return
	end
	while restart_server.pop
	server_thread = server.run

The PR contains two small fixes.

1. Prevent server from starting if worker receives TERM signal (0018edd). This handles some (but not 100% of all) edge cases where the TERM signal is received and calls server.stop before the server has started.
2. Send TERM to fork_worker child processes in Cluster#wait_workers (aae4d4d) - When fork_worker is used, the call to Process.wait in wait_workers returns ECHILD for forked workers. This commit adds a call to w.term in the rescue clause, matching the behavior when fork_worker is not used and ensuring workers shut down properly. This should handle all other edge-cases where a worker receives TERM before the server starts.

Finally, note that this bug started occurring more frequently after #2435, because that PR changed behavior so that calling #stop before #run now has no effect. I've added test_command_ignored_before_run to clarify this new behavior in a unit test. I think this is more intuitive and predictable behavior overall, but we can change it back if anyone disagrees or if the change continues to cause any other issues.