Refactor GetEpochMeta API

[lizhefeng]

EpochMeta now contains:

1. EpochData(epoch number, epoch start height, gravity chain start height);
2. Total Number of Blocks;
3. []BlockProducerInfo (Address, Votes, Active or Not, Block Productions)

Comment out all the delegate related part in ioctl for now, as it needs to be refactored accordingly.

[codecov]

Codecov Report

Merging #848 into master will decrease coverage by 0.02%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #848      +/-   ##
==========================================
- Coverage   58.76%   58.73%   -0.03%     
==========================================
  Files         149      149              
  Lines       12263    12267       +4     
==========================================
- Hits         7206     7205       -1     
- Misses       4093     4097       +4     
- Partials      964      965       +1

Impacted Files	Coverage Δ
blockchain/blockchain.go	48.22% <0%> (ø)	⬆️
action/protocol/poll/protocol.go	19.49% <0%> (-0.43%)	⬇️
api/api.go	64.02% <80%> (-0.09%)	⬇️

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