Refactor s3client ReadFile interface to return a channel.

[niedbalski]

Refactor the ReadFiles interface method to be ReadFile and return a channel of []string instead
of passing the channel as a reference in the method.

Also, handles the case of dynamic buffer size configured to handle scanning errors for long lines
on files.

Log the error condition when scanning fails due to a shorter buffer to ensure user is aware
on debug mode.

Adds test to cover it.