glog should use ThreadSanitizer (TSAN) dynamic annotations about glog

[toddlipcon]

Another example in stacktrace_libunwind-inl.h -- the 'g_now_entering' variable should be stored using a 'Release_Store' at the end of GetStackTrace(...).

Probably many more, so I won't bother listing them all.

I know that Google uses TSAN internally heavily. Does Google have any intention to open source newer versions of glog, or should we assume that any improvements here will have to be done by the external community?

[toddlipcon]

One more: there should be an ANNOTATE_BENIGN_RACE on fatal_msg_data_shared to prevent a TSAN warning when multiple threads FATAL at the same time. (or it should be made thread-safe)

[kennyyu]

@toddlipcon This PR will mark the LOG_EVERY_N macros as benign races: #263