[profiler] Initialize threads in runtime initialization callback.

[profiler] Initialize threads in runtime initialization callback.

For the helper and writer thread to actually work, we need Mono's threading
subsystem to be initialized (so we can use TLS and hazard pointers). We now
wait with starting these threads until Mono gives us the go-ahead.

This means that a bit more data will be buffered up in the thread-local log
buffers during startup, but it shouldn't really matter in practice.