This reverts commit cc6201b0db.
On Android the loggers are not always initialized when several .so are
used, leading to crashes. It's not completely clear what is exactly
happening.
- libutil's slog, cout, cerr, cwarn and cinfo were not thread safe
at all.
- they are now thread_local
- for that reason the log buffer (which now exists per thread) is not
allocated until it's needed and is aggressively shrunk.
- removed our thread_local emulation which was incomplete.
- don't expose LogStream publicly.
