Fixes bug with excessive rate limiting

[farazdagi]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

• Fixes issue when rate limiting was too aggressive, depleting capacity excessively
• Improves speed of test (15s -> 5s), but setting up database for blocks used in tests only (not all blocks in range)

Which issues(s) does this PR fix?

Issue located when working on init-sync optimizations, and fix was found immediately - so no corresponding issue opened.

Other notes for review
We had several issues:

• Rate limiter's capacity was decreased even before any block streaming occurred - which is unfair, as before blocks are streamed, we can't count them. It is correct to check if limits are exceeded before sending anything, but updating capacity should occur after the streaming.
• More importantly, we've always decreased capacity by allowedBlocksPerSecond:
  

  prysm/beacon-chain/sync/rpc_beacon_blocks_by_range.go

  Line 78 in 34c02ff

  r.blocksRateLimiter.Add(stream.Conn().RemotePeer().String(), int64(allowedBlocksPerSecond))

  
  This means, that with our current init-sync routine, where requests are interleaved, and it is common to request less than a dozen of blocks for each peer, every request (even for, say, 4 blocks) - will be considered a served request for 64 blocks. So, effectively, sometimes we mark as bad peers, peers which are not bad at all. That's our node sends 9 requests for 4 blocks (and it keeps track to make sure it doesn't get over the limit) in 1 sec - it gets banned (we do not allow x10 increase - x9 is enough to get banned).

This issue shouldn't manifest itself with update (#5798) where I indeed force single peer to fetch close to allowed max blocks in each request. But with current routine - we are excessively banning peers.

[codecov]

Codecov Report

Merging #5853 into master will decrease coverage by 0.34%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5853      +/-   ##
==========================================
- Coverage   60.31%   59.96%   -0.35%     
==========================================
  Files         312      312              
  Lines       27158    26190     -968     
==========================================
- Hits        16379    15705     -674     
+ Misses       8605     8349     -256     
+ Partials     2174     2136      -38