Handle the return values of realtime updates and stores more accurately.

Realtime backends' update and store callbacks return the number of rows affected,
or -1 if there was a failure. There were a couple of issues:

* The config API was treating 0 as a successful return, and positive values as
  a failure. Now the config API treats anything >= 0 as a success.

* res_sorcery_realtime was treating 0 as a successful return from the store
  procedure, and any positive values as a failure. Now sorcery treats anything
  > 0 as a success. It still considers 0 a "failure" since there is no change
  to report to observers.

Review: https://reviewboard.asterisk.org/r/3341



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@410592 f38db490-d61c-443f-a65b-d21fe96a405b