Set expiry time on uref

New urefs will expire by default- lifetime must be set on creation. Lifetime is number of eras as cost will be set according to the number of eras.
Expiry is at end of era, when it is pruned if expired. Allows more data to be purged. Note: EE doesn't have a concept of 'era' so it will be a forward calculation of block-time.

Start by locating where urefs are created and where expired data is pruned, then compare era-based expiry with the forward block-time calculation described for EE. Done means lifetime is required at creation, expiry is evaluated at the intended boundary, and expired data can be purged consistently.

Written by the indexing model from the issue text.