daemon/config: cache open by default (even if not in config)

before the cache was disabled by default, but this has led to many user
errors (mine as well). this enables it by default (which is what most
people want anyway)

daemon/lua/config.lua

--- Default configuration
-cache.open(10*MB)
 -- Listen on localhost
 if not next(net.list()) then
 	if not pcall(net.listen, '127.0.0.1') then

daemon/lua/sandbox.lua

@@ -66,6 +66,8 @@ setmetatable(cache, {
 		else   rawset(t, k, v) end
 	end
 })
+-- Defaults
+cache.size = 10 * MB
 
 -- Syntactic sugar for TA store
 trust_anchors = kres.trust_anchors