logging: add support for streaming operators

logging: add support for streaming operators

Summary:
Update the logging library so that FB_LOG() and FB_LOGF() also accept
streaming style arguments:

  FB_LOG(logger) << "logs can be written like this now: " << 1234;
  FB_LOG(logger, "both styles can be used") << " together";
  FB_LOGF(logger, "test: {}, {}", 1234, 5678) << " and with FB_LOGF() too";

Streaming support does make the upcoming XLOG() macros more complicated to
implement, since the macro has to expand to a single ternary `?` expression.
However I was able to come up with a solution that works and has minimal
overhead.

Reviewed By: wez

Differential Revision: D5082979

fbshipit-source-id: 11734e39c02ad28aceb44bbfbd137d781caa30cf