fix#440
WARNING
multi_buffer constructor now takes a maximum size instead of the
"allocation size". This is a breaking change. The allocation size
feature is removed. To update calling code, remove the allocation
size parameter from call sites.
* multi_buffer now uses a geometric growth algorithm for better performance.
The concept type traits are renamed for consistency,
and consolidated into a single header file <beast/core/type_traits.hpp>
A new section, Core Concepts, is added to the documentation describing all
of the core utility classes and functions. This also includes a complete
explanation and sample program describing how to write asynchronous initiation
functions and their associated composed operations.
