clarified server start, notably for invocation within regular Isabelle/Scala process;

--- a/src/Pure/Tools/server.scala	Fri Mar 09 12:45:53 2018 +0100
+++ b/src/Pure/Tools/server.scala	Fri Mar 09 13:03:55 2018 +0100
@@ -134,6 +134,7 @@
               stmt.execute()
             })
 
+            server.start
             (entry, Some(server))
         }
       })
@@ -258,6 +259,8 @@
       }
     }
 
+  def start { server_thread }
+
   def join { server_thread.join; shutdown() }
 
   def shutdown() { server_socket.close }