Use slf4j instead of log4j to support other logging backends

Logback users are getting a [multiple bindings warning]((http://www.slf4j.org/codes.html#multiple_bindings).  This removes the hard dependency on log4j:

> Embedded components such as libraries or frameworks should not declare a dependency on any SLF4J binding but only depend on slf4j-api. When a library declares a compile-time dependency on a SLF4J binding, it imposes that binding on the end-user

Keeps log4j as the internal logging implementation for datastores' tests.