Implement List pagination in S3 backend

[yiranwang52]

Add pagination parameters to backend interface:
https://github.com/uber/kraken/blob/master/lib/backend/client.go
And update the logic accordingly for the S3 implementation.

[surapuramakhil]

Can I work on it?
just for clarification, a List data structure where objects are stored in S3 (simple storage service).
can you assign it? in order to avoid confits of two pull request for the same issue :)

[yiranwang52]

We made some progress on this issue, there is a list implementation, but it doesn't support pagination. You are welcome to contribute :)

Also I am having trouble assigning issues to users not already in the project. Don't worry, I doubt there will be conflicts.

[surapuramakhil]

@yiranwang52 is it in master? if not can you share the branch 😄

[surapuramakhil]

just for concept clarification. splitting a huge file into smaller parts and parts are connected through the list. am I right?

[yiranwang52]

There could be multiple objects under a path, and List return a list of names with given prefix.