Request handling improvements

[alcjzk]

Main changes

• Implemented a validating class for request content-length (enforces HTTP grammar for the value)
• Added HTTP status Content Too Large
• Request handling now uses separate buffersizes for headers / body, both from the configuration
• Added a Connection class to carry information throughout lifetime of a client connection (keep-alive is now contained here).
• Refactored everything to use the connection context class instead of the contained separate values.

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Connection

The class' main purpose is to carry any state that needs to live for the full duration of the connection. This includes the following:

        File                  _client;
        const Server&         _server;
        bool                  _keep_alive = true;
        std::optional<Reader> _reader = std::nullopt;

Value for _reader will retain any reader (readbuffer) state that needs to persist beyond a single request. This fixes issues when a single recv call reads more than one request into the buffer.

All of these values have also been so far passed separately into basically all tasks, so the refactor to use this as replacement is quite large.

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Other changes

• Removed legacy FileResponse
• Various helper utilities into Reader & Buffer
• Fixed #57

Not in this PR

Some todo items will be handled outside this PR, due to the large refactor.

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