Enhancement/lp refactor download

[lpoli]

Changes

Following changes are added:

1. Add reader that complies with io.ReadSeekCloser interface
2. Provide download function for zboxcli to read file into some file; it also serves as an example to how to use reader

Fixes

Tests

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• Merge your system tests PR to master AFTER merging this PR

Associated PRs (Link as appropriate):

• blobber:
• 0chain:
• system_test:
• zboxcli: Add read command zboxcli#405
• zwalletcli:
• Other: ...

[peterlimg]

Not sure what does the mod do here. And seems it could cause data lose. Imagine we request block_1, the offset is 109, and effectiveChunkSize is 100, then we will copy the data[9:] in block_1 to b. So where's the data[:9] going.

[lpoli]

@peterlimg If client requests with offset 109, then it means client does not want or care about data[:9]. So for example, if client already had stored 109 bytes of data then for requesting other data offset would be set to 109. Since data is requested in block level(100 bytes in above example), 109 bytes means client has stored 100 bytes from block_0 and 9 bytes from block_1, so now after getting block_1, it should skip data[:9] otherwise data will be corrupted.

[peterlimg]

LGTM