Add a Puma plugin for Solid Queue

This allows you to run Solid Queue as part of a puma cluster, by adding

```
plugin :solid_queue
```

to your Puma config.

It will boot up the Solid Queue supervisor, leaving it to manage the
scheduler and worker processes as normal.

If either Puma or the Solid Cache supervisor dies, the other will notice
and shut itself down.

On puma shutdown, it will signal Solid Queue to also shutdown.

lib/puma/plugin/solid_queue.rb

@@ -0,0 +1,63 @@
+require "puma/plugin"
+
+Puma::Plugin.create do
+  attr_reader :puma_pid, :solid_queue_pid, :log_writer
+
+  def start(launcher)
+    @log_writer = launcher.log_writer
+    @puma_pid = $$
+    @solid_queue_pid = fork do
+      Thread.new { monitor_puma }
+      SolidQueue::Supervisor.start(mode: :all)
+    end
+
+    launcher.events.on_stopped { stop_solid_queue }
+
+    in_background do
+      monitor_solid_queue
+    end
+  end
+
+  private
+    def stop_solid_queue
+      Process.waitpid(solid_queue_pid, Process::WNOHANG)
+      log "Stopping Solid Queue..."
+      Process.kill(:INT, solid_queue_pid) if solid_queue_pid
+      Process.wait(solid_queue_pid)
+    rescue Errno::ECHILD, Errno::ESRCH
+    end
+
+    def monitor_puma
+      monitor(:puma_dead?, "Detected Puma has gone away, stopping Solid Queue...")
+    end
+
+    def monitor_solid_queue
+      monitor(:solid_queue_dead?, "Detected Solid Queue has gone away, stopping Puma...")
+    end
+
+    def monitor(process_dead, message)
+      loop do
+        if send(process_dead)
+          log message
+          Process.kill(:INT, $$)
+          break
+        end
+        sleep 2
+      end
+    end
+
+    def solid_queue_dead?
+      Process.waitpid(solid_queue_pid, Process::WNOHANG)
+      false
+    rescue Errno::ECHILD, Errno::ESRCH
+      true
+    end
+
+    def puma_dead?
+      Process.ppid != puma_pid
+    end
+
+    def log(...)
+      log_writer.log(...)
+    end
+end