Mini refactoring: using std::size_t and std::numeric_limits

Created by: knowledge4igor

I've done mini refactoring:

1. I replaced using size_t to std::size from C++11 standard;
2. Introduced using std::numeric_limits<std::size_t>::max() instead of castings -1.

Also asio::streambuf size method returns std::size(). I think it would be better to use std::size for consistency.

I will be glad if you consider this pull request.