don't ignore address_book cache

don't need to preload users that are already in the cache. not
functionality change, just performance

test-plan: N/A

Change-Id: I34aecd97a35619718a7c66c902974d3b2cec09ae
Reviewed-on: https://gerrit.instructure.com/109287
Reviewed-by: Jonathan Featherstone <jfeatherstone@instructure.com>
Tested-by: Jenkins
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>

lib/address_book/service.rb

@@ -114,6 +114,8 @@ module AddressBook
     end
 
     def preload_users(users)
+      return if users.empty?
+
       # make sure we're dealing with user objects
       users = hydrate(users) unless users.first.is_a?(User)
 

lib/api/v1/conversation.rb

@@ -73,8 +73,9 @@ module Api::V1::Conversation
   # ensure the common contexts for those users are fetched and cached in
   # bulk, if not already done
   def preload_common_contexts(current_user, recipients)
-    users = recipients.select{ |recipient| recipient.is_a?(User) }
-    current_user.address_book.preload_users(users)
+    address_book = current_user.address_book
+    users = recipients.select{ |recipient| recipient.is_a?(User) && !address_book.cached?(recipient) }
+    address_book.preload_users(users)
   end
 
   def conversation_recipients_json(recipients, current_user, session)