Refactor to use common method for checking process liveness

Adds critical clarifying comments about the meaning of signal 0, which
is effectively a liveness/existence (and permissions) check
See http://man7.org/linux/man-pages/man2/kill.2.html for a discussion
of that

ee/lib/gitlab/sidekiq_cluster.rb

@@ -137,7 +137,7 @@ module Gitlab
     # Returns true if all the processes are alive.
     def self.all_alive?(pids)
       pids.each do |pid|
-        return false unless signal(pid, 0)
+        return false unless process_alive?(pid)
       end
 
       true
@@ -148,7 +148,13 @@ module Gitlab
     end
 
     def self.pids_alive(pids)
-      pids.select { |pid| signal(pid, 0) }
+      pids.select { |pid| process_alive?(pid) }
+    end
+
+    def self.process_alive?(pid)
+      # Signal 0 tests whether the process exists and we have access to send signals
+      # but is otherwise a noop (doesn't actually send a signal to the process)
+      signal(pid, 0)
     end
 
     def self.write_pid(path)