dnsdist: Prevent implicit conversions between dnsdist::Protocol and uint8_t

dnsdist: Prevent implicit conversions between dnsdist::Protocol and uint8_t

This commit makes sure that we always use the dnsdist::Protocol type,
and that we cannot easily convert to or from a different type by mistake.

C++ really dropped the ball by making it impossible to declare methods
on the 'enum class' that solved the issue of making regular enums
implicitly convertible to int and back, thus making it possible to
assign and compare different types of enums together, as well as
enums and ints.
The result is that we are stuck with declaring classes to hold our
methods, along with a lot of plumbing to make sure that we can convert
in some cases but not allow obvious mistakes from happening.