Align to spec v0.9

[terencechain]

With the release of v0.9 to facilitate new shading proposal. This PR tracks to the progress to align Prysm to v.0.9 which is important for multi client test net. The experimental branch for this is: #3760

The v.0.9 consists of the following:

Beacon chain

feature

• alter beacon proposer selection logic (Improve beacon proposer selection logic ethereum/consensus-specs#1413)
• add seed domains (Add seed domains ethereum/consensus-specs#1415)
• fix delay-based attestation inclusion reward (Fix delay-based attestation inclusion reward ethereum/consensus-specs#1434)
• use eth1_block_hash for initial randao_mix (Implement solution for #1446, based on suggested use of eth1 hash ethereum/consensus-specs#1447)

simplifications

• remove active active_index_roots and compact_committees_roots from phase 0 (Remove light client infrastructure from phase 0 ethereum/consensus-specs#1329)
• remove shards/crosslinks from phase 0 (Remove Shards/Crosslinks from Phase 0 ethereum/consensus-specs#1428)
• remove transfers from phase 0 (Remove transfers from phase 0 ethereum/consensus-specs#1443)

Fork choice

• verify state root in fork choice tests and other minor fixes (Forkchoice tests fixed ethereum/consensus-specs#1454)

Validator

• add simple attestation aggregation strategy (Simple aggregation strategy ethereum/consensus-specs#1440)
• remove validator API and migrate to eth2 APIs repo (remove Val API and add links to outside spec/standards repos ethereum/consensus-specs#1366)

Networking

• add simple attestation aggregation strategy (Simple aggregation strategy ethereum/consensus-specs#1440)

Simple Serialize

• define summaries and expansions (Add summaries and expansions to simple-serialize.md ethereum/consensus-specs#1360)
• add clarifications on deserialization (SSZ clarifications on deserialization ethereum/consensus-specs#1381)
• update to v0.9 version of the spec in the go-ssz repo

Spec test

• slot processing
• block processing
• epoch processing
• proto compatibility
• ssz

Run time

• one node with 64 validators
• two nodes with 128 validators each

[terencechain]

Working on beacon proposer selection logic and remove shards/crosslinks

[terencechain]

Closed via #4098