On OSX, Lean was often crashing when using trace messages.
I identified a problem in the thread finalization process.
In OSX, the `silent_ios_helper` at `library/trace.cpp` was being
finalized after the `null_streambuf` at `util/null_ostream.cpp`.
There was also a memory corruption problem also related to
`null_streambuf`.
This commit fixes this problem by using the following recipe
for creating null output stream buffers in C++.
https://stackoverflow.com/questions/11826554/standard-no-op-output-stream
