Amend basic_buffer's reserve concept (to be the same as std::string).
Enhance the default packer and unpacker to support basic_buffer as their output message type.
Enhance packer2 and unpacker2 to support unique_buffer<basic_buffer> and shared_buffer<basic_buffer> as their output message type.
To use the original default packer and unpacker, now you must write them as packer<> and unpacker<>.