Refactor some code of Input{CSV,UDPSocket}::get_next_request() into
InputMedium::get_next_request().
The medium-specific code must now be implemented in
fill_current_request().
- Where possible, use initialisation lists in class definition instead
of explicit variable initialisations in source file.
- Rename some variables (use of "source" in copy constructors and
operators…).
- Suppress useless Doxygen comments, allow non-documented members in
Doxyfile.