Simplify BeaconGossipHandler

[dapplion]

• Store forkDigest in an enum state variable
• Subscribe to all core topics on start
• Sync calls start() instead of handleSyncComplete(), it's the sync instance responsibility to know that gossip must start when completing sync not the other way around

[codeclimate]

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[dapplion]

The sim tests fail because some of the nodes don't reach justification. Some of them fall behind randomly and are unable to keep up.

Test run 1

[NODE 0]           info: Synced - finalized: 0 0x0000…0000 - head: 7 0x3aa0…008a - peers: 3
[NODE 2]           info: Syncing - 25 seconds left - 0.200 slots/s - finalized: 0 0x0000…0000 - head: 3 0x52fe…4d95 - currentSlot: 8 - peers: 3
[NODE 1]           info: Synced - finalized: 0 0x0000…0000 - head: 7 0x3aa0…008a - peers: 3
[NODE 3]           info: Synced - finalized: 0 0x0000…0000 - head: 1 0xe9d5…68be - peers: 3

Test run 2

[NODE 3]           info: Synced - finalized: 0 0x0000…0000 - head: 1 0xacbe…cf57 - peers: 3
[NODE 0]           info: Synced - finalized: 0 0x0000…0000 - head: 23 0xcd5c…faac - peers: 3
[NODE 1]           info: Synced - finalized: 0 0x0000…0000 - head: 23 0xcd5c…faac - peers: 3
[NODE 2]           info: Synced - finalized: 0 0x0000…0000 - head: 23 0xcd5c…faac - peers: 3