Fix deadlock in GetRewardsAccumulated

[hyeonLewis]

Proposed changes

This PR is on top of #48

GetRewardsAccumulated has deadlock at reqCh<-num when len(errCh)>numWorkers since non-nil error closes goroutine. This PR makes separate goroutine for requests.

Console test

// Before PR
> governance.getRewardsAccumulated(700, 701)
Error: missing trie node e58fbf31ca5b4a4c5e2435f37fa0e87bbec954f1900d2f4da9810eab9b39214b (path )
	at web3.js:6812:9(39)
	at send (web3.js:5223:62(29))
	at <eval>:1:33(4)

> governance.getRewardsAccumulated(700, 1700)
...
^ deadlock occurs

// After PR
> governance.getRewardsAccumulated(700, 1700)
Error: missing trie node 31371ed81cf6c6d0df48d2b5d61414a6e695aa6390f45fef50290d3d13295021 (path )
	at web3.js:6812:9(39)
	at send (web3.js:5223:62(29))
	at <eval>:1:33(4)

> governance.getRewardsAccumulated(700, 701)
Error: missing trie node e58fbf31ca5b4a4c5e2435f37fa0e87bbec954f1900d2f4da9810eab9b39214b (path )
	at web3.js:6812:9(39)
	at send (web3.js:5223:62(29))
	at <eval>:1:33(4)

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[blukat29]

@yoomee1313 @ian0371 PTAL