Switch mysql-proxy to simpleproxy

Mysql-proxy turns out too unreliable and unstable for use in
production. The packaged version on Ubuntu suffers from a critical DOS
by using telnet. This patch switches from mysql-proxy to simpleproxy,
which is just a tcp proxy to forward incoming port 3306 connections to
the subunit2sql db.

Change-Id: Iffea64aea46cc34969bbaa970e5d91bd0cc05232