Epoch Transition at Start of Epoch - (Spec PR #732, #741)

[schroedingerscode]

Description

As an implementor, I want to transition epochs at the start of the epoch instead of during the last slot of the epoch, so that off-by-one issues related to transition helpers functions and verification are mitigated.

Acceptance Criteria

• Transition logic is reordered as defined in 732 linked below.
• Remove off-by-one kludge during attestation processing in the per-block processing logic.
• Cache block root in initial state cache to ensure that block root of previous slot is available for epoch processing.
• Remove setting state_root during advance_slot portion of per-epoch processing.

Additional Information

See ethereum/consensus-specs#732
See ethereum/consensus-specs#741