refactor s3api/utils into more meaningful sub package names

[benmcclelland]

Describe the solution you'd like
See this post on why "utils" is not the best package name:
https://dave.cheney.net/2019/01/08/avoid-package-names-like-base-util-or-common

Additional context
We have in there:

• auth-reader
• chunk-reader
• csum-reader
• logger
• presign-auth-reader
• utils

We could probably split this up into sub packages of s3api that better stand on their own. Most of the functions in utils.go should probably just be moved to the calling package and unexported.