Fetches only a small portion of the message for previews: Closes #4254, Closes #3799

Before we were fetching the entire message body (including attachments) to get the
preview text.  This patch now offers the ability to fetch a small (128 byte) preview
of the email.

Also, since this ticket is about speeding up performance, I've introduced NonblockingBatch,
which allows for multiple async operations to be executed in parallel easily.  I've added
its use in a few places to speed up operations, including one that was causing the lag
in #3799, which is why this commit closes that ticket.

sql/Create.sql

@@ -39,7 +39,9 @@ CREATE TABLE MessageTable (
     
     header TEXT,
     
-    body TEXT
+    body TEXT,
+    
+    preview TEXT
 );
 
 CREATE INDEX MessageTableMessageIDIndex ON MessageTable(message_id);