Ticket Support

This is the implementation of tickets, as described in https://ripple.com/wiki/Multisign, which is required for M-of-N work.

A set of integrations tests to check ticket creation, lookup and deletion will be needed and will be pushed into this request as soon as they are done. 
I don't quite see the point of these methods we have everywhere - why not just call make_unique<CancelTicket>(txn, params, engine) when you need it?  More cruft!