SPARK-3782 [CORE] Direct use of log4j in AkkaUtils interferes with certain logging configurations

Although the underlying issue can I think be solved by having user code use slf4j 1.7.6+, it might be helpful and consistent to update Spark's slf4j too. I see no reason to believe it would be incompatible with other 1.7.x releases: http://www.slf4j.org/news.html  Lots of different version of slf4j are in use in the wild and anecdotally I have never seen an issue mixing them.

Author: Sean Owen <sowen@cloudera.com>

Closes #4184 from srowen/SPARK-3782 and squashes the following commits:

5608d28 [Sean Owen] Update slf4j to 1.7.10