eth: Backfill blocks in batches

[leszko]

What does this pull request do? Explain your changes. (required)
Optimize the number of ETH RPC request by first backfilling blocks in batches during the block sync. In result, only the latest block is polled separately instead of polling all missing blocks one-by-one.

This also fixes the Livepeer downtime issue in the case when Livepeer cannot catch up with the blocks and therefore can't transcode.

Specific updates (required)

• In syncToLatestBlock(), execute first block backfilling and then poll block only for the latest one
• Update unit tests

How did you test each of these updates (required)

• Deployed an orchestrator and checked that it can transcode the video
• Check that block backfilling catches all interesting information (e.g. DepositFund, etc.)

Does this pull request close any open issues?

fix #2480

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[codecov]

Codecov Report

Merging #2489 (a68c4ef) into master (3230eb1) will decrease coverage by 0.02841%.
The diff coverage is 28.57143%.

@@                 Coverage Diff                 @@
##              master       #2489         +/-   ##
===================================================
- Coverage   54.73000%   54.70159%   -0.02841%     
===================================================
  Files             95          95                 
  Lines          19852       19855          +3     
===================================================
- Hits           10865       10861          -4     
- Misses          8383        8390          +7     
  Partials         604         604                 

Impacted Files	Coverage Δ
cmd/livepeer/starter/starter.go	2.29665% <0.00000%> (-0.00661%)	⬇️
eth/blockwatch/block_watcher.go	51.54867% <36.36364%> (-0.88496%)	⬇️

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[red-0ne]

This also fixes the Livepeer downtime issue in the case when Livepeer cannot catch up with the blocks and therefore can't transcode.

Is this solved by the fact that sync is faster because of fewer RPC calls?

[red-0ne]

Have a couple of questions. Other than that, it looks fine for me.

[leszko]

This also fixes the Livepeer downtime issue in the case when Livepeer cannot catch up with the blocks and therefore can't transcode.

Is this solved by the fact that sync is faster because of fewer RPC calls?

Right, it's solved, because in many cases Livepeer will never catch up with blocks polling them one by one, so it's effectively down. Using backfilling, it will most likely catch with the missing blocks.

[red-0ne]

Looks good to me.