🗑️ Deprecate client_thread with a warning

There is no one single client_thread, this isn't used internally, and
it's misleading to external users.  However, adding `#reciever_thread`
(or `#receiver_fiber`) might be useful.

Author: nevans

lib/net/imap.rb

@@ -734,8 +734,6 @@ class IMAP < Protocol
     # Seconds to wait until an IDLE response is received.
     attr_reader :idle_response_timeout
 
-    attr_accessor :client_thread # :nodoc:
-
     # The hostname this client connected to
     attr_reader :host
 
@@ -768,6 +766,11 @@ class << self
       alias default_ssl_port default_tls_port
     end
 
+    def client_thread # :nodoc:
+      warn "Net::IMAP#client_thread is deprecated and will be removed soon."
+      @client_thread
+    end
+
     # Disconnects from the server.
     #
     # Related: #logout