Fix: function send_msg still not visible after defined macro ASCS_EXPOSE_SEND_INTERFACE.

If both macro ASCS_PASSIVE_RECV and ASCS_SYNC_RECV been defined, the first invocation of recv_msg will be omitted too.

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.