♻️ Tweak connection state change in receiver thread

This pushes `ensure state_logout!` from the receiver's `Thread.start`
down into `#receive_responses`.  As a side-effect, this also pulls
the state change inside the receiver thread's exception handling.  But
that's fine: it fits better inside `receive_responses`, and
`state_logout!` really should never raise an exception anyway.

Author: nevans

lib/net/imap.rb

@@ -3350,8 +3350,6 @@ def start_receiver_thread
       rescue Exception => ex
         @receiver_thread_exception = ex
         # don't exit the thread with an exception
-      ensure
-        state_logout!
       end
     end
 
@@ -3433,6 +3431,8 @@ def receive_responses
           @idle_done_cond.signal
         end
       end
+    ensure
+      state_logout!
     end
 
     def get_tagged_response(tag, cmd, timeout = nil)