Fixes/build warnings

[ikmsk10]

The PR solves the #104

• Fix inherit base classes without virtual destructors
• Prevent -Wunused-parameter warning.

[eddic]

Hey Igor. I'm just getting to this now. Thanks for this pull request. This is very well done. A couple questions first though:

• The [[maybe_unused]] attribute. This is listed as C++17. The fastcgi++ project is C++14 at this point. I'm a little worried about this but unsure. Will this effect backwards compatibility?
• The virtual destructors you've added. For most of them the need is evident. Some, however, I am unclear on. I've always been a little leery about creating vtables for classes that aren't intended to be used polymorphically. Most of these already have vtables but I believe some do not. What are your thoughts on this?