Movable HTTP messages, parsers, serializers, and corresponding stream algorithms

beast use a lot of L value ref as parameter, sometimes this makes it really painful to design my class

for example:

`http:async_write` takes either a `message&` or `serializer&`, which means something else need to hold the value until the callback is done.

but very often I generated this message or serializer in function body that doesn't own any data structure to store it. this force me to redesign the class, many lines of code are added to make sure messages are kept in memory until callback is called

if functions like `async_write` can have a version to accept a R value ref and store it internally. my class would be much simpler and easy to read