Ticket #801 implementation requiring specific enabling of Down Evolutions

https://play.lighthouseapp.com/projects/82401/tickets/801-ups-only-mode-for-evolutions

This is implemented as requiring Down evolutions to be turned on explicitly with `-DapplyDownEvolutions.db=true` , and Up-Only evolutions to function with the existing `-DapplyEvolutions.db=true`

evolutions docs updated. Also added a missing section on Evolutions with Multihosts using Locking.