network plugin: Use the meta data to implement the `Forward' option.

network plugin: Use the meta data to implement the `Forward' option.

Previously, a cache in the network plugin was used to keep track of
which values were received via the network in order to distinguish
between ``forwarded'' values and values that were received from
somewhere else.

The same cache was also used to avoid loops when forwarding packages by
keeping track of the highest timestamp that was sent by the plugin and
discard received data that was older or as old as that.

This information is not kept in the meta data of the global cache (what
is the last timestamp sent) and the meta data of the value list (was
this value list received via the network?). The cache that was
maintained in the network plugin has been removed.