Enhance SLF4J use inside Zookeeper.

[César Álvarez Núñez]

Hi all,

I would like to:

   - Modify the usage of slf4j api on zk code since calls to
   org.slf4j.Logger methods should use parameters instead of "string
   concatenation" (http://www.slf4j.org/faq.html#logging_performance)
   - Update to latest SLF4J version.

But I have several questions:

   1. There is no need to apply ZOOKEEPER-850 to zookeeper-trunk since it
   is already done. Right?
   2. It should be managed as a new jira or as an evolution of
   ZOOKEEPER-850?
   3. Since there will be many affected files, should be open a new branch?

Since I'm a beginner contributor any suggestion/feedback is welcome.

BR,
/César.