Summary:
GLOG has `-logbuflevel` option that allows a user to specify the level above which none of the logs should be buffered. This is helpful as we can configure it such that `INFO` and all verbose logs are buffered before being output, but all logs above `INFO` should be flushed synchronously. This will ensure that all logs above a certain level are printed before any sort of crash.

This diff allows a user to specify a config `sync_level`. The value for this can be any `LogLevel`. Example config:

  .=WARN,akkio.cli=INFO; default:async=true,sync_level=WARN

The default value for GLOG's -logbuflevel is `0` (`INFO`).
To match GLOG's behavior, this diff sets the following as defaults:

  async=true,sync_level=WARN

Reviewed By: simpkins

Differential Revision: D8867555

fbshipit-source-id: 7ec30dfb41b2f3cd3568d70304db7a9fcf668779