Renaming counter_update() to more intuitive counter_add()

Change-Id: I57b6e74ecca7f0f89d0e974a8af229627c638e48
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4217
Reviewed-by: Ippokratis Pandis <ipandis@cloudera.com>
Tested-by: jenkins