Align to spec v0.9.1

[terencechain]

With the release of v0.91 to remove custody bit construct and modify fork choice rule to harden defenses. This PR tracks to the progress to align Prysm to v.0.91

The v.0.91 consists of the following:

Beacon chain

• remove custody_bits from Phase 0(Remove custody_bits and AttestationDataAndCustodyBit ethereum/consensus-specs#1462)

Fork choice

• add "bounce-attack" resistance (Bounce attack resistance ethereum/consensus-specs#1465)
• add "decoy-flip-flop" resistance(Decoy flip flop resistance ethereum/consensus-specs#1466)

Validator

• minor clarification for range of included deposits in genesis(Specify inclusive range for genesis deposits ethereum/consensus-specs#1460)
• fix period_tail in get_eth1_vote(Fix period tail in get_eth1_vote ethereum/consensus-specs#1468)
• clarify previous_eth1_distance for get_eth1_vote(Clarify previous_eth1_distance ethereum/consensus-specs#1469)

Misc

• Unskip all the spec tests

[terencechain]

thanks @prestonvanloon for taking on fix period_tail in get_eth1_vote

[prestonvanloon]

Remaining open issues are ETH1Data voting related, tracked here: #4117