tsan: don't initialize trace header in release mode

We are going to use only a small part of the trace with the default
value of history_size. However, the constructor writes to the whole trace.
It writes mostly zeros, so freshly mmaped memory will do.
The only non-zero field if mutex type used for debugging.

Reduces per-goroutine overhead by 8K.

https://code.google.com/p/thread-sanitizer/issues/detail?id=89

llvm-svn: 229127