[FEATURE] Add `continuous download` feature

[cnlangzi]

for now, we have a feature dowload by blocks. but you have to calculate blockNums and blockIndex, and loop it till done. download has to restart from scratch if network is broken. A new continuous download should be added

• automatically download it with blocks
• continuous download from last breakpoint when network is back or ./zbox download is restarted
• parallel download all blocks from all blobbers

related to https://github.com/0chain/gosdk/issues/314

[sculptex]

I don't understand:-

• parallel download all blocks from all blobbers

Each chunk is already downloaded in parallel, adding additional instances of multiple chunks is self-defeating purpose of continuous/partial download

[lpoli]

Since this is related to https://github.com/0chain/gosdk/issues/314, I am closing it here.