Kong: config changes propagate to other nodes on a poll interval

# kong.conf
db_update_frequency = 5        # seconds between cache refreshes
db_update_propagation = 0      # extra delay for eventually-consistent stores
db_cache_ttl = 0               # 0 = cache forever until invalidated

# force one node to forget everything
curl -X POST http://localhost:8001/cache/purge

# Timeline after a PATCH on one node, with db_update_frequency=5:
#   t+0    node A serves the new config
#   t+0-5  node B still serves the old one
#   t+5    node B polls and catches up
#
# In DB-less mode there is no propagation at all - each node must be given
# the new file, which is why /config is usually driven by a deployment
# rather than by hand.