Persist string attributes over the TCP stream

Currently, only numeric attributes are persisted, while string attributes are saved in strings.properties. Since string attributes are not persisted, this means they are not sent over the TCP stream. We would like for string attributes to be sent over the TCP stream just as numeric attibutes are, as many useful OIDs (such as SLA/RPM test names) are strings. This can be controlled by a property so that it does not affect normal operation where string as written in strings.properties.