Use variadic templates in ns-3

The goal of this issue is to track the progress of migrating to variadic templates.

The relevant contributions are:

• !147 (merged)
• https://www.nsnam.org/bugzilla/show_bug.cgi?id=2457 (partially included in the above)
• https://www.nsnam.org/bugzilla/show_bug.cgi?id=2780
• https://www.nsnam.org/bugzilla/show_bug.cgi?id=3012
• !276 (merged)
• !278 (merged)
• !734 (closed)

The only remaining issue is MakeEvent (see below comment thread).