IOTEX-349 Reenable unit tests of ReadActiveBlockProducersByHeight and ReadCommitteeBlockProducersByHeight

[lizhefeng]

1. Port hotfix of r0.5 on master
2. Reenable API tests on reading active BPs and committee BPs

[codecov]

Codecov Report

Merging #770 into master will increase coverage by 0.04%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #770      +/-   ##
==========================================
+ Coverage   59.59%   59.63%   +0.04%     
==========================================
  Files         133      133              
  Lines       11486    11484       -2     
==========================================
+ Hits         6845     6849       +4     
+ Misses       3756     3751       -5     
+ Partials      885      884       -1

Impacted Files	Coverage Δ
server/itx/server.go	0% <0%> (ø)	⬆️
action/protocol/poll/protocol.go	20.9% <33.33%> (+0.72%)	⬆️
blockchain/blockchain.go	51.17% <0%> (+0.07%)	⬆️
consensus/scheme/rolldpos/rolldposctx.go	75.57% <0%> (+0.17%)	⬆️
db/trie/extensionnode.go	59.32% <0%> (+1.69%)	⬆️

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