store dhtput data locally so it will be refreshed like the other dht entries we maintain.
should help with missing DHT entries (issue #165), although i think #165 would benefit of a more agressive (faster) retrying of remote stores.
truncate file when saving settings
keep track of dht_storage_item which have been locally added.
for now this is used to make sure all kinds of resources (including rts and replies)
will have a forced refresh for at least two days.