Solve blocking read of incomplete RPCs (simple DoS)

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Now the library return within timeout only when no data is available. However, 
if an attacker sent only partial data (without the RPC-end-sequence), the 
nc_session_rpc_recv() function will get stuck on read - denial of service.

To solve this problem, add a buffer of read but not yet processed data to every 
session. If the data will turn out to be incomplete, return from 
nc_session_recv_rpc() within the specified timeout and keep the contents of the 
buffer for a later append of the rest of the message.

Original issue reported on code.google.com by mv6...@gmail.com on 18 Dec 2014 at 10:07