MULE-8146: Grizzly thread leaks

Changed GrizzlyServerManager so that the transport is initialized when the first server is registered. Otherwise, Grizzly threads are created just by having the module as a dependency, even if no listener-config is defined in the application.
Merge pull request #1306 from mulesoft/MULE-8146

MULE-8146: Grizzly thread leaks