use category specific logging instead of log.msg()

lib/carbon/log has helper methods that let you record the identity of
the component that is issuing a log statement. This requires the
application code to invoke log.foo() as opposed to log.msg() where __foo__
can be one of the predefined names such as _relay_, _cache_, _query_ etc. etc.

While large parts of the application do use these more specialized
methods, there exists some part of the code base that does not use these.
This patch fixes up all such invocations