Move debugging state and some methods to new GdbServer class.
Move diverter code to replayer.cc.
Remove vestigial diverter files.
Rename 'session' to 'diversion_session'
Move diversion variables/code into GdbServer.
Move dispatch_debugger_request into GdbServer.
Move replay_one_step into GdbServer.
Remove session parameter from replay_one_step.
Remove static gdb_server.
Make most GdbServer members private.
Move GdbServer to GdbServer.h/.cc.
Move gdb_rr_macros to GdbServer.cc
Move is_ignored_replay_signal to ReplaySession.
Move start_debug_server to GdbServer::emergency_debug.
Make GdbServer::process_debugger_requests private.
Make GdbServer::serve_replay_with_debugger a static GdbServer::serve function.
Trim headers in replayer.cc.
Rename GdbContext to GdbConnection.
Move dbg and debugger_active into GdbServer.
Make restart_session take const GdbRequest&.