Better duplicate detection in local database

This incorporates much better duplicate detection in the local database, using both RFC-822 Message-ID as well as IMAP metadata (internaldate, RFC822 size) to determine if a message is already stored in the database.  Very useful when a message is stored in multiple folders, or an already-downloaded message is returned to a folder it originated in (i.e. INBOX).

Also some minor fixes to listing email by EmailIdentifier which save a roundtrip to the server for certain edge cases.

src/engine/sqlite/email/sqlite-message-table.vala

@@ -70,6 +70,8 @@ public class Geary.Sqlite.MessageTable : Geary.Sqlite.Table {
         return id;
     }
     
+    // TODO: This could be improved greatly, in particular making this a single SQL command or
+    // parallelizing the commands.
     public async void merge_async(Transaction? transaction, MessageRow row,
         Cancellable? cancellable) throws Error {
         Transaction locked = yield obtain_lock_async(transaction, "MessageTable.merge_async",