ports: handler_wrapper: use std::ref instead of lambda and add comment.
ports: handler_wrapper: use std::ref instead of lambda and add comment.

A std::ref object is as good a forwarding lambda. The context of why the
handler_wrapper was written is explained in a comment block before the
functions. tldr: lvalues may be noncopyable, need to be stored in
something copyable to work with std::function.

FlexCoreLib/flexcore,CasparKielwein/flexcore