logging: add a LoggerDB::internalWarning() function

logging: add a LoggerDB::internalWarning() function

Summary:
A few places in the logging code need to report issues when something goes
wrong in the logging code itself.  This adds a new LoggerDB::internalWarning()
method for use in these places.

This provides a default implementation that logs these warnings with
_CrtDbgReport() on Windows, and writes to stderr on non-Windows platforms.
The default implementation can also be overridden for programs that want some
alternative behavior.

Reviewed By: wez

Differential Revision: D5157731

fbshipit-source-id: e27f5587d58b3941fca8d5b38c8c3edbcb75d7f3