Implemented a reply socket, which means that there is now
a RPC Server with 0MQ!

It's running in a seperate thread.

All rpc command(s) should work, since we are calling the same
functions which the current RPC Server is using.

An example in Lua is under contrib/0mq/bitcoin-zmq_reqrep.lua.

Start with:

'bitcoind -zmqrepbind="ipc://.bitcoin.rep"'

and then run the example client.