conversations: sort thread by createdmodseq if same internaldate

This gives us a stable sort when multiple external messages are appended
very quickly, giving them the same internaldate.

This is being added without a new version number, as it extends a data
structure in a backwards and forwards compatible way.  If not set, the
CREATEDMODSEQ field will be zero, which is fine.  This will still lead
to correct sorting for newly added emails on any thread.  The only
slight weirdness might be if an existing message in a thread gets moved
or copied into a new folder, which will lead to it getting a
createdmodseq value, and hence sorting to the end for that internaldate.
This will be very rare, and a convdb rebuild will fix it.

imap/conversations.c

@@ -960,6 +960,7 @@ static void conv_to_buf(conversation_t *conv, struct buf *buf, int flagcount)
         dlist_setguid(nn, "GUID", &thread->guid);
         dlist_setnum32(nn, "EXISTS", thread->exists);
         dlist_setnum32(nn, "INTERNALDATE", thread->internaldate);
+        dlist_setnum32(nn, "CREATEDMODSEQ", thread->createdmodseq);
     }
 
     dlist_setnum64(dl, "CREATEDMODSEQ", conv->createdmodseq);
@@ -1489,6 +1490,11 @@ int _saxconvparse(int type, struct dlistsax_data *d)
             rock->thread->internaldate = atol(d->data);
             rock->substate = 3;
             return 0;
+
+        case 3:
+            rock->thread->createdmodseq = atoll(d->data);
+            rock->substate = 4;
+            return 0;
         }
         return 0; // there might be following fields that we ignore here
 
@@ -1771,6 +1777,11 @@ static int _thread_datesort(const void **a, const void **b)
     if (r < 0) return -1;
     if (r > 0) return 1;
 
+    // if same internaldate, use createdmodseq for a stable sort
+    int64_t r2 = (ta->createdmodseq - tb->createdmodseq);
+    if (r2 < 0) return -1;
+    if (r2 > 0) return 1;
+
     return message_guid_cmp(&ta->guid, &tb->guid);
 }
 
@@ -1804,6 +1815,7 @@ static void conversations_thread_sort(conversation_t *conv)
 static void conversation_update_thread(conversation_t *conv,
                                        const struct message_guid *guid,
                                        time_t internaldate,
+				       modseq_t createdmodseq,
                                        int delta_exists)
 {
     conv_thread_t *thread, **nextp = &conv->thread;
@@ -1829,7 +1841,15 @@ static void conversation_update_thread(conversation_t *conv,
     }
 
     message_guid_copy(&thread->guid, guid);
-    thread->internaldate = internaldate;
+    // these should always be the same for all copies of an email!
+    // but if not (e.g. IMAP append) we want the earliest non-zero value
+    if (!thread->internaldate || thread->internaldate > internaldate)
+        thread->internaldate = internaldate;
+    // the same email may exist multiple times in a folder or in multiple
+    // folders with different createdmodseq.  We want to track the earliest
+    // one
+    if (!thread->createdmodseq || thread->createdmodseq > createdmodseq)
+        thread->createdmodseq = createdmodseq;
     _apply_delta(&thread->exists, delta_exists);
 
     conversations_thread_sort(conv);
@@ -2585,6 +2605,7 @@ EXPORTED int conversations_update_record(struct conversations_state *cstate,
     conversation_update_thread(conv,
                                &record->guid,
                                record->internaldate,
+                               record->createdmodseq,
                                delta_exists);
 
     r = conversation_update(cstate, conv, &ecounts,

imap/conversations.h

@@ -125,6 +125,7 @@ struct conv_thread {
     struct message_guid guid;
     uint32_t exists;
     time_t internaldate;
+    modseq_t createdmodseq;
 };
 
 struct conv_folder {