Implement HeaderByHeight in filedao

[Liuhaai]

What would you like to be added

HeaderByHeight, which to be implemented in filedao_v2, should be used instead of GetBlockByHeight in

iotex-core/blockchain/filedao/filedao.go

Line 235 in 1791f82

blk, err := v2.GetBlockByHeight(height)

// Before 
blk, err := v2.GetBlockByHeight(height)
// After 
header, err := v2.HeaderByHeight(height)

Why is this needed

Some queries can be optmized by HeaderByHeight

• iotex-core/api/web3server.go

  Line 850 in 1791f82

  blkStores, err := svr.coreService.BlockByHeightRange(filterObj.LogHeight, queryCount)

• iotex-core/chainservice/builder.go

  Line 672 in 1791f82

  blk, err := dao.GetBlockByHeight(height)

How important you think this is for IoTeX

• [] must have
• [] should have
• [] nice to have

Additional information

Let us know any background or context that would help us better understand the request (for example the particular use-case that prompted this request)

[millken]

Currently HeaderByHeight still needs to deseralize the entire block, the cost is the same as GetBlockByHeight.

[Liuhaai]

Currently HeaderByHeight still needs to deseralize the entire block, the cost is the same as GetBlockByHeight.

I see no HeaderByHeight is implemented in filedao_v2. Could you give a snippet why should the whole block be deseralized?