[logger] use trace for low-level tracing info (followup)

[sebsto]

As I can't reopen #129
There are still info level messages that are left.

Here is a proposed PR
#132

As per https://www.swift.org/documentation/server/guides/libraries/log-levels.html, best practices for libraries is to use debug and trace only to let the developer (user of the library) to choose the verbosity level they provide to their customers