Fixed compilation warnings on Linux

There were two compiler warnings (see below). The firewall.cc fix switches from <ctype> to <chrono>. The acknowledgement.cc fix uses decltype, and I am not too proud of it, but it does the trick.

/home/breese/src/maidsafe/maidsafe/src/routing/src/maidsafe/routing/firewall.cc:59:63: error: comparison between signed and unsigned
 integer expressions [-Werror=sign-compare]                                                                                        
         return (lhs.birth_time - rhs.birth_time < Parameters::firewall_message_life_in_seconds);

/home/breese/src/maidsafe/maidsafe/src/routing/src/maidsafe/routing/acknowledgement.cc:190:27: error: comparison between signed and 
unsigned integer expressions [-Werror=sign-compare]                                                                                
                     }) == expected) {