Unfreeze threads for some object-evaluating commands

[st0012]

There are several commands that evaluate objects and call methods on them, such as:

• info
• ls (outline)
• trace object
• display

If the called method acquires a mutex shared with another thread (e.g. when using Timeout), then it'd cause a deadlock as all threads are stopped.

For example, if there's an ActiveRecord Relation object stored as a local variable, and the user runs the info command, then it'd call inspect on it and trigger a database query, it could cause a deadlock as described in #877.

This commit fixes the issue by unfreezing all threads before evaluating those commands and freezing them again after receiving the result event.

Note: theoretically there are more commands that evaluate user program's objects, like break Foo#bar calls Foo.instance_method. But in that case the problem would only arise when Foo.instance_method is overridden with something that'd acquire the lock, which is a far unlikely to happen.

[ko1]

I'm not sure it is safe because current thread management technique does not have enough quality to control them, but try it.