TACHYON-102 Change log to use slf4j.
Change LOG.debug to use SLF4J parameterized message to reduce String concat.

Remove toString on log to use parameterized logging.

Remove toString on log to use parameterized logging.

Revert style format change in core/pom.xml.
Small format fix on pom.xml file.
Fix the import ordering for org.slf4j.Logger
Merge pull request #440 from hsaputra/TACHYON-102_updatelog_slf4j

Tachyon-102 Move logging to SLF4J (with log4j as backend implementation)
Tachyon-102  Replace leftover log4j to slf4j from merging other PRs.

Also updated the how to contribute doc to make sure we document this.

Remove extra newline.

Fix text
Merge pull request #442 from hsaputra/missing_log4j_to_slf4j

Tachyon-102  Replace leftover log4j to slf4j from merging other PRs.