Zero-day tasks to arrange into separate tickets

[Gerold103]

Create tickets for the following items and discard the not needed ones.

• Introduce socket options and allow to pass them to mg::aio::TCPServer and mg::aio::TCPSocket. Allow to set which of them are critical;
• Introduce const SharedPtr;
• Introduce UDPSocket;
• Introduce SSLSocket based on OpenSSL (#5);
• Introduce SSLSocket based on mbedTLS;
• Introduce HTTPClient (#22);
• Use uint32_t in Error.h for all codes, not int;
• Make IPV6 optional and cover it with tests;
• Introduce destructor of all globals including DomainToIP;
• Perform IOCore benchmarks same way as the others, via report.py, and save the results for public (#8);
• Introduce mg::net create and destroy, for WSAStartup;
• Allow to destroy IOCore while having sockets in it. They should all get a special on-close event;
• Introduce mini buffers;
• Make RecvCommit in TCPSocket shrink the recv buffer if it is too much bigger than recvsize;
• Make TCPSocketIface::Delete more explicit with its own callback;