Don't print profiling information twice

Don't print profiling information twice

It's important to `.clear` the messages so that we don't print them
again for the next "profiling session". It might be argued that really
we should have a single "profiling session" for Cargo, but we don't at
the moment.

And, while we are at it, let's lock stdout as well, so that we won't
get confused when Cargo becomes multi-threaded and prints profiling info
from several threads simultaneously.