In various places we have locks and lock-protected variables, we should use https://github.com/abseil/abseil-cpp/blob/master/absl/base/thread_annotations.h to provide hints to the compiler and sanitizers for GUARDED_BY etc.

There's some investigation to be done to find a solution that works cleanly on Clang OS X, see #2527 for example. Otherwise, it mostly works today and we can use these annotations once OS X is resolved.

We should switch the style guide to require them for new code, and then add missing annotations to existing code once this is in place.