Replace AddrInfoGuard with finally

[xiaozhuai]

1. AddrInfoGuard is bound to addrinfo, it's not reuseable.
2. AddrInfoGuard did not impl the move constructor.
3. AddrInfoGuard should mark copy constructor as delete.
4. finally is simple, clean, safer, and higher reuseable.
5. finally macro implement something like golang's defer keyword, but it will exec when leaving current scope, not function.
6. Taken from https://gist.github.com/xiaozhuai/7b33e100f4f30918215e736a755d6df9

[Be3y4uu-K0T]

Why not just use a smart pointer (unique_ptr)?

std::unique_ptr<addrinfo, decltype(&freeaddrinfo)> finally(server_info, &freeaddrinfo);

[netcan]

Why not just use a smart pointer (unique_ptr)?

std::unique_ptr<addrinfo, decltype(&freeaddrinfo)> finally(server_info, &freeaddrinfo);

~the memory allocation is not neccessary.~ it's semantic doesn't make sense, sounds tricky.

[xiaozhuai]

Why not just use a smart pointer (unique_ptr)?

std::unique_ptr<addrinfo, decltype(&freeaddrinfo)> finally(server_info, &freeaddrinfo);

Using a unique_ptr can't do this (multiple clean up)

finally {
    delete[] aaa;
    free(bbb);
    freeaddrinfo(info);
};

And in this case, unlock a mutex, If using a unique_ptr, it would be more complex and tricky.

finally {
    mutex.unlock();
};

And also, using lambda is better for compiler to do (inline) optimization.