Optimized Ethereum beacon light client and E2E test

[hujw77]

• An optimized Ethereum beacon light client

// Current arthitecture diverges from spec's proposed updated splitting them into:
// - Finalized header updates: To import a recent finalized header signed by a known sync committee by `import_finalized_header`.
// - Sync period updates: To advance to the next committee by `import_next_sync_committee`.
//
// To stay synced to the current sync period it needs:
// - Get finalized_header_update and sync_period_update at least once per period.
//
// To get light-client best finalized update at period N:
// - Fetch best finalized block's sync_aggregate_header in period N
// - Fetch parent_block/attested_block by sync_aggregate_header's parent_root
// - Fetch finalized_checkpoint_root and finalized_checkpoint_root_witness in attested_block
// - Fetch finalized_header by finalized_checkpoint_root
//
// - sync_aggregate -> parent_block/attested_block -> finalized_checkpoint -> finalized_header
//
// To get light-client sync period update at period N:
// - Fetch the finalized_header in light-client
// - Fetch the finalized_block by finalized_header.slot
// - Fetch next_sync_committee and next_sync_committee_witness in finalized_block
//
// - finalized_header -> next_sync_committee
//
// ```
//                       Finalized               Block   Sync
//                       Checkpoint              Header  Aggreate
// ----------------------|-----------------------|-------|---------> time
//                        <---------------------   <----
//                         finalizes               signs
// ```
//
// To initialize, it needs:
// - BLS verify contract
// - Trust finalized_header
// - current_sync_committee of the trust finalized_header
// - genesis_validators_root of genesis state
//
// When to trigger a committee update sync:
//
//  period 0         period 1         period 2
// -|----------------|----------------|----------------|-> time
//              | now
//               - active current_sync_committee
//               - known next_sync_committee, signed by current_sync_committee
//
//
// next_sync_committee can be imported at any time of the period, not strictly at the period borders.
// - No need to query for period 0 next_sync_committee until the end of period 0
// - After the import next_sync_committee of period 0, populate period 1's committee

• E2E test for Ethereum beacon light client

[hackfisher]

LGTM