Avoid locking all threads on debugger suspension

Details in Shopify/team-ruby-dx#724

This commit simply doesn't lock any threads during suspension, which
is what byebug does. It may prevent users from investigating threading
problem, but it's still better than freezing the process just by running
a ActiveRecord query.

Author: st0012

lib/debug/session.rb

@@ -1686,7 +1686,7 @@ def get_thread_client th = Thread.current
         DEBUGGER__.debug{ "Enter subsession (nested #{@subsession_stack.size})" }
       else
         DEBUGGER__.debug{ "Enter subsession" }
-        stop_all_threads
+        # stop_all_threads
         @process_group.lock
       end