NPM cache: migrate ddb to s3

[edgarpoce]

Goal
migrate npm cache registry entry from ddb to s3

Implementation

• removed ddb table
• now the same bucket is used to store both the registry entry and the version tarballs. index is stored under the library folder with index.gz as the file name. There shouldn't be any collision because tarballs are stored with .tgz as the extension.
• refactored lambdas in the scope of this work with a more object oriented design
• added unit tests
• added seamless migration support. when autoupdate is executed the registry entry is stored in s3.

[troyready]

Great start. Added my nitpicky thoughts inline.

[troyready]

@edgarpoce I neglected to include this in my last review as well: some of the top-level README needs to be updated to reflect these changes

[edgarpoce]

@edgarpoce I neglected to include this in my last review as well: some of the top-level README needs to be updated to reflect these changes

fixed in d5e4c96