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<>.

Simplify class basic_buffer (doesn't support capacity concept anymore).
flexible_unpacker support basic_buffer as its message type.

Add flexible_unpacker--an unpacker which support huge messages but with a limited pre-allocated buffer.

A small optimization for message sending (use message replication rather than parsing message).
Drop direct_broadcast_msg, use broadcast_native_msg instead.