Allow configuring modules dynamically without process restart

Today in order have a filter module or router module configuration to take into effect, a user have to

• update MaxScale.cnf
  -restart maxscale

which means all the existing client and server connections get dropped.

In production environment this is not usable approach to have to restart MaxScale for every filter configuration change. A mechanism to dynamically change filter or any other module configuration without restarting maxscale is needed.

This came across when @Kolbe Kegel tried to change firewall filter rules and reload it.

Proposal is t that "each plugin has its own set of actions and information that should be exposed via an API. Some plugins might want to expose and implement a "reload" method so that their configuration can be reloaded."

Along with this corresponding maxadmin command should also be added.