Avoid locking all threads on debugger suspension (#16)

This commit stops locking all threads during suspension, which may prevent
users from investigating threading problem. But it's still better than
freezing the process just by running a ActiveRecord query, or doing
anything that calls `Timeout.timeout`.

History around this issue:

1. It originally was reported in Shopify/team-ruby-dx#724,
   and we reported it upstream too.
1. We introduced 1e0f45c as a workaround.
1. Upstream proposed a solution to fix the query evaluation case by restarting
   threads before evaluating console/DAP input. So the above commit was
   reverted.
1. However, the process could still hang if `Timeout.timeout` is called:
     1. During a suspension
     2. And outside of input evaluation
   An example is typing Arrow-up in the console as `reline` calls `Timeout.timeout`
   underneath.
1. So we need to reintroduce the workaround until we can find a better
   with the upstream maintainer.

lib/debug/session.rb

@@ -1698,7 +1698,7 @@ 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