Support multiple endpoints per socket

Sent messages will be scheduled among the endpoints according to socket
type: PUB will send the same data to all endpoints simultaneously, PUSH
will do round robin transfer.
Incoming data is fair queued between endpoints.

This is a feature of at least zeromq and nanomsg.

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To use: in the device configuration, instead of specifying just one address,
specify a comma separated list e.g.

tcp://localhost:123,ipc:///tmp/socket

the connection method (bind/connect) applies to all endpoints in this case.
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Mixing binding and connecting endpoints is supported:
prefix "@" means "bind", "+" (or ">") means connect, e.g.

+tcp://localhost:123,@ipc:///tmp/socket,ipc:///tmp/asd

(in case of missing prefix, the default channel method is used for that
endpoint).

fairmq/FairMQSocket.h

@@ -37,6 +37,7 @@ class FairMQSocket
 
     virtual bool Bind(const std::string& address) = 0;
     virtual void Connect(const std::string& address) = 0;
+    virtual bool Attach(const std::string& address, bool serverish = false);
 
     virtual int Send(FairMQMessage* msg, const std::string& flag = "") = 0;
     virtual int Send(FairMQMessage* msg, const int flags = 0) = 0;