Tachyon-102 Move logging to SLF4J (with log4j as backend implementation)

This PR contains work for sub-tasks under Tachyon-102:
1. TACHYON-104 Update logger to use SLF4J
2. TACHYON-105 Update debug string concat to use SLF4J parameterized feature

I punted upgrade to Log4J 2 to separate issue [1] since we have custom Appender which needs a bit more work to move to Log4J 2.

[1] https://tachyon.atlassian.net/browse/TACHYON-107
wrong import order.