Support a read-only backend db

[dustin]

The DB API currently only supports one db_url. We should support two, so that most of the getXxx methods can use the read-only DB. Operations that require read-after-write consistency would still use the read-write DB.

MySQL supports master/slave replication, where the slave(s) can support read-only operations, so this would allow us to scale database access appropriately for read-heavy applications (e.g., frontend web servers). MySQL replication has a small delay, so we'd still want to do things like claiming buildrequests exclusively on the read-write DB.

[dustin]

This is not terribly difficult, but I don't know of anyone who needs it just yet.

[dustin]

db modifications should be done in nine at this point.