Slave build ID should not be used for much at all

The slave build ID is not trustworthy -- anybody can easily compromise a slave and make it return whatever build ID they want. But it's currently used in lots of buildd-manager logging, which is confusing and potentially unsafe.

Nothing should touch slave build IDs except dispatchBuildToSlave, rescueIfLost and verifySlaveBuildID. Once all users have been adjusted to not use it, Builder.slaveStatus should be taught to stop putting it in the dict.