[WIP] [SPARK-2655] Change logging level from INFO to DEBUG

Currently, it's too noisy at the default level INFO, so it's better to put the logging about runtime details into DEBUG level.

Basic rule is that, DEBUG is used for developer or debugging, INFO is used for user (who does not know much of the details). So if some loggings do not provide helpful messages for users, it should be put in DEBUG level.

The progress of Job/stage/task is helpful for user, should be keep in INFO level. (It will be better to put all messages of task completeness into one line, not addressed in this PR)

The logging level in third-parties should be set to WARN by default.
I'd keep all the ones in SparkContext in INFO. I think they are kind of important ...