logtostderr filtering

[guliashvili]

Currently, glog supports filtering of the logs using stderrthreshold only when the logtostderr and alsologtostderr are False.

However, when logtostderr is not true, glog creates the log files, which for our use case is unnecessary, as we have the sink for writing in the file.

I propose creating either flag to disable logging in the file or stderrthreshold_forced flag, which would filter things when logtostderr is true.

I'm happy to invest some time in that and would love to hear feedback from you.