Add in lock mechanism for downloading from origin

[chetanddesai]

Many times we run multiple stateless node processes. When multiple processes get a cache miss, we need to lock so both don't start a download from s3.

[chetanddesai]

• What happens when the process crashes when there is a lock and you're downloading?
• Always download to a different directory and then link it. https://nodejs.org/api/fs.html#fs_fs_linksync_srcpath_dstpath
• https://github.com/npm/lockfile/blob/master/lockfile.js