MULE-7870: Be able to provide socket configuration for the HTTP connector

Renamed properties and added default values after code review.
Null has the same meaning as in the rest of the attributes that don't have defaults: the attribute is not set in the config and thus should not be explicitly set in the library that is being used (therefore using the library or OS default). It's true that it can't be set in XML, but it can eventually be set programatically.