Move sent_messages callbacks into transmit_message().

This mostly sets the stage for a subsequent commit to start
using client_message_id as the key into sent_messages.

It has the nice side effect of making it more explicit that
certain things should always happen when transmit_message()
succeeds.

This commit does regress our node test coverage a bit.

tools/test-js-with-node

@@ -47,7 +47,6 @@ enforce_fully_covered = {
     'static/js/recent_senders.js',
     'static/js/rtl.js',
     'static/js/search_suggestion.js',
-    'static/js/sent_messages.js',
     'static/js/stream_events.js',
     'static/js/stream_sort.js',
     'static/js/topic_generator.js',