Allow configuring automatic saving rate on map_server

[corot]

That is, do not save on any map callback. When making a map with gmapping it saves a new make every 5 s (default publish rate on gmapping), what crowds the database innecessarily. Worse is that map chooser map selection menu gets crowded with half-done useless maps. I propose two possible changes:

• Add a parameter with maximum saving rate, what is a trivial change.
• Overwrite temporal map during current session, what requires a bit more work but is cleaner and more elegant, I think.