Enable SQLite secure_delete pragma.

By default, when a row is deleted, its contents are preserved until its space is requested for a new insertion. This makes possible to extract the contents from deleted messages, even the ones encrypted and self-destroyed.

When SQLite has its secure_delete pragma activated and a row is deleted, it overwrites its contents with zeros, making harder to recover the text.

Thanks to Abel Gomez from INCIDE for noticing this (http://atrapadosporlosbits.blogspot.com.es/2014/02/destruccion-de-mensajes-en-telegram.html).