Refactor: Modernizing filters to Cpp11.

[Dimi1010]

Part of #977

The change is mostly C-style casts to Cpp style casts.
Marked all parseToString overrides as override.

Additionally, some changes I think it might be beneficial, but I would like an opinion before starting on them:

1. parseToString being marked as const.
   Reasoning: The conversion to string should not modify the filter internals under any circumstances.
   Potential issues: If someone has written custom filters inherited from GenericFilter or down the chain and implemented their own parseToString this will be a breaking change for the custom filter as parseToString and parseToString const are considered different functions by the compiler.
   No expected issues with plain usage of the function, if only one version exists, or the non-const version calls the const version as a default via some black magics like:

virtual void parseToString(std::string &result)
{
    static_cast<GeneralFilter const *>(this)->parseToString(result);
};
virtual void parseToString(std::string &result) const = 0;

2. enum -> enum class - Read some of the discussion in Refactor code: use enum class in TcpLayer #1289, about potential solutions to that. Duplicated enums/ctors seem like the best solution for keeping the backwards compatibility in the intermediary period. Although I am debating on the naming. Simplest solution is to stick a V2 or Type suffix or something? (example Direction -> DirectionV2 or DirectionType).
   Also I am wondering what to do with the functions that return an enum like Direction getDir()?

3. Implement C++11 move semantics for filter ctors that take string const&. Either by implementing 'perfect forwarding' or 'pass by value and move' as a simpler alternative.

4. Explicitly defaulting empty destructors. Should produce no compatibility issues in the current situation but can have some caveats in other situations explained here

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.38%. Comparing base (28b9072) to head (7c72afb).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1323      +/-   ##
==========================================
- Coverage   82.41%   82.38%   -0.03%     
==========================================
  Files         163      163              
  Lines       20938    20938              
  Branches     7903     7903              
==========================================
- Hits        17256    17250       -6     
- Misses       3019     3026       +7     
+ Partials      663      662       -1     

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