Set @tc ivar the moment tc is received

We should pass tc (ThreadClient)'s data in the Session in a uniformed way:
either all with ivar or all with local variable. And since the local variable
approach has been rejects (#293), we may want to go with @ivar.

This change for unification is needed for later changes.

Author: st0012

lib/debug/session.rb

@@ -204,7 +204,7 @@ def session_server_main
 
     def process_event evt
       # variable `@internal_info` is only used for test
-      tc, output, ev, @internal_info, *ev_args = evt
+      @tc, output, ev, @internal_info, *ev_args = evt
 
       capture_tui_data if @ui.tui?
 
@@ -219,20 +219,20 @@ def process_event evt
         q << true
 
       when :init
-        wait_command_loop tc
+        wait_command_loop
 
       when :load
         iseq, src = ev_args
         on_load iseq, src
         @ui.event :load
-        tc << :continue
+        @tc << :continue
 
       when :trace
         trace_id, msg = ev_args
         if t = @tracers.values.find{|t| t.object_id == trace_id}
           t.puts msg
         end
-        tc << :continue
+        @tc << :continue
 
       when :suspend
         enter_subsession if ev_args.first != :replay
@@ -242,22 +242,22 @@ def process_event evt
         when :breakpoint
           bp, i = bp_index ev_args[1]
           clean_bps unless bp
-          @ui.event :suspend_bp, i, bp, tc.id
+          @ui.event :suspend_bp, i, bp, @tc.id
         when :trap
-          @ui.event :suspend_trap, sig = ev_args[1], tc.id
+          @ui.event :suspend_trap, sig = ev_args[1], @tc.id
 
           if sig == :SIGINT && (@intercepted_sigint_cmd.kind_of?(Proc) || @intercepted_sigint_cmd.kind_of?(String))
             @ui.puts "#{@intercepted_sigint_cmd.inspect} is registered as SIGINT handler."
             @ui.puts "`sigint` command execute it."
           end
         else
-          @ui.event :suspended, tc.id
+          @ui.event :suspended, @tc.id
         end
 
         if @displays.empty?
-          wait_command_loop tc
+          wait_command_loop
         else
-          tc << [:eval, :display, @displays]
+          @tc << [:eval, :display, @displays]
         end
       when :result
         raise "[BUG] not in subsession" unless @subsession
@@ -289,14 +289,14 @@ def process_event evt
           # ignore
         end
 
-        wait_command_loop tc
+        wait_command_loop
 
       when :dap_result
         dap_event ev_args # server.rb
-        wait_command_loop tc
+        wait_command_loop
       when :cdp_result
         cdp_event ev_args
-        wait_command_loop tc
+        wait_command_loop
       end
     end
 
@@ -329,9 +329,7 @@ def inspect
       "DEBUGGER__::SESSION"
     end
 
-    def wait_command_loop tc
-      @tc = tc
-
+    def wait_command_loop
       loop do
         case wait_command
         when :retry