Add some basic memcached handling to status_network

Add some basic memcached handling to status_network

Status_network can't be a subclass of Memcached_DataObject -- the
latter is too entrenched in Laconica's memc handling functions, which
aren't loaded when Status_network is running! But the importance of
caching these values can't be overstated. So, a considerably
slimmed-down version of the Memcached_DataObject code is transcribed
into Status_network.