MINOR: trace: make trace() now also take a level in argument

MINOR: trace: make trace() now also take a level in argument

This new "level" argument will allow the trace sources to label the
traces for different purposes, and filter out some of them if they
are not relevant to the current target. Right now we have 5 different
levels:
  - USER : the least verbose one, only a few functional information
  - PAYLOAD: like user but also displays some payload-related information
  - PROTO: focuses on the protocol's framing
  - STATE: also indicate state internal transitions or non-transitions
  - DEVELOPER: adds extra info about branches taken in the code (break
    points, return points)