Align to spec v0.9.2

[terencechain]

With the release of v0.9.2. This issue tracks to the progress to align Prysm to v.0.9.2

The v0.9.2 consists of the following:

Aggregation Refactor

We need to refactor entirely how aggregation works based on ethereum/consensus-specs#1484

Beacon chain

• remove Hash alias and add Root alias for Bytes32(Avoid Hash <> hash() type and func confusion ethereum/consensus-specs#1478)
• reduce minimum genesis validator count in mainnet config (reduce genesis validator count for mainnet config ethereum/consensus-specs#1467)
• Use public validator API methods defined in https://github.com/prysmaticlabs/ethereumapis
• Use custom state SSZ (not required for restart, can be rolled out)

Fork choice

• harden fork choice to a few scenarios (Forkchoice mods ethereum/consensus-specs#1477)

Validator

• provide explicit directives to validators on which attestation topics to join and when(Phase 0 attestation topic directives ethereum/consensus-specs#1476)

[rauljordan]

All items done