cache delegators for validators for previous era to avoid expensive re-lookups.

[RitaMAllenCA]

In 2.0 delegators are stored under a composite key and resolution of the delegators

Security hardening opportunity.

[devendran-m]

Design conversation in the in nxt sprint.

[devendran-m]

Potential future optimisation.

[EdHastingsCasperAssociation]

Removing this optimization from the Epic as it is deferred, is not a release blocker, is orthogonal to disk storage, and should not block completion of feature.