Define a clear Log Level Policy for Leshan.

[sbernard31]

Following discussion #1634 (comment), it seems that we maybe need a clear Log Level Policy for Leshan.

Currently we haven't really clear guideline for log level usage, see https://github.com/eclipse-leshan/leshan/wiki/Code-&-design-guidelines#code-rules.

We probably need to clarify that maybe searching if there is reference practice about it ? (https://stackoverflow.com/questions/5817738/how-to-use-log-levels-in-java)
Ideally this policy should define the log level to use about "client bad behavior"

Let's use this issue to discuss about it.