Fix stringstream crash during shutdown

[duke8253]

Seeing crashes during shutdown with this stack, removing stringstream looks like the only choice.

[ 00 ] libc-2.17.so                  raise                                     
[ 01 ] libc-2.17.so                  __GI_abort                                
[ 02 ] libstdc++.so.6.0.19           __gnu_cxx::__verbose_terminate_handler()  
[ 03 ] libstdc++.so.6.0.19                                                     
[ 04 ] libstdc++.so.6.0.19           std::terminate()                          
[ 05 ] libstdc++.so.6.0.19           __gxx_personality_v0                      
[ 06 ] libgcc_s-4.8.5-20150702.so.1                                            
[ 07 ] libgcc_s-4.8.5-20150702.so.1  _Unwind_ForcedUnwind                      
[ 08 ] libpthread-2.17.so            __GI___pthread_unwind                     
[ 09 ] libpthread-2.17.so            sigcancel_handler                         
[ 10 ] libpthread-2.17.so                                                      
[ 11 ] libstdc++.so.6.0.19           std::locale::~locale()                    
[ 12 ] asf_ssl_session_reuse.so      hex_str(std::string)                      ( basic_ios.h:282 )
[ 13 ] asf_ssl_session_reuse.so      RedisSubscriber::run()                    ( subscriber.cc:182 )
[ 14 ] asf_ssl_session_reuse.so      setup_subscriber(void*)                   ( subscriber.cc:47 )
[ 15 ] traffic_server                ink_thread_trampoline                     ( InkIOCoreAPI.cc:128 )
[ 16 ] libpthread-2.17.so            start_thread                              

[shinrich]

Cleans up messy shutdown crashes.

[zwoop]

Cherry-picked to v9.0.x branch.