a message serialization support

[wuyueandrew]

Is your feature request related to a problem? Please describe.
My cpp_pregel algo needs to send a structed message and now it is not supported.
My solution is connecting these field using concat, it's not elegant.

Describe the solution you'd like
a specify message serialization , like flatbuffers which can offer a encapsulation of structed message.

[yecol]

You may want to implement the serialization/deserialization for your structure in InArchive/OutArchive..