metrics_dump: attribute "handystats.dump_timestamp" replaced with attributes "handystats.system_timestamp" and "handystats.internal_timestamp".

Attribute "handystats.system_timestamp" represents unix timestamp in milliseconds to which current dump is related.
Attribute "handystats.internal_timestamp" represents unix timestamp in milliseconds to which metrics' state is related.
Almost always these timestamps will be the same except high income load, when core will not be able to process all messages timely.