refactor(runloop) do not select Services/Routes with nulls

When reading from the proxy, we do not wish to retrieve unset columns as
`ngx.null` for backwards-compatibility reasons. That is now the default
for all plugins since 735314c (parent commit), for plugins, and it is
now for the runloop as well.

Context: retrieving unset columns as `nil` instead of `ngx.null` allows
for backwards-compatibility with existing plugins. An example of this is
authentication plugins retrieving the authenticated Consumer, and
injecting its fields as headers into the upstream request (e.g.
`X-Consumer-Username`). When such fields are unset, the following
headers were sent upstream: `X-Consumer-Username: userdata: NULL`
instead of no header at all (or worse, producing HTTP 500 errors).

kong/runloop/handler.lua

@@ -28,7 +28,6 @@ local fmt         = string.format
 local sort        = table.sort
 local ngx         = ngx
 local log         = ngx.log
-local null        = ngx.null
 local ngx_now     = ngx.now
 local update_time = ngx.update_time
 local re_match    = ngx.re.match
@@ -86,7 +85,7 @@ end
 
 local function build_router(db, version)
   local routes, i = {}, 0
-  local routes_iterator = db.routes:each(nil, { nulls = true })
+  local routes_iterator = db.routes:each()
 
   local route, err = routes_iterator()
   while route do
@@ -99,7 +98,7 @@ local function build_router(db, version)
     local service
 
     -- TODO: db requests in loop, problem or not
-    service, err = db.services:select(service_pk, { nulls = true })
+    service, err = db.services:select(service_pk)
     if not service then
       return nil, "could not find service for route (" .. route.id .. "): " .. err
     end
@@ -109,7 +108,7 @@ local function build_router(db, version)
       service = service,
     }
 
-    if route.hosts ~= null then
+    if route.hosts then
       -- TODO: headers should probably be moved to route
       r.headers = {
         host = route.hosts,
@@ -543,7 +542,7 @@ return {
       -- TODO: this is probably not optimal
       do
         local retries = service.retries or api.retries
-        if retries ~= null then
+        if retries then
           balancer_data.retries = retries
 
         else
@@ -552,7 +551,7 @@ return {
 
         local connect_timeout = service.connect_timeout or
                                 api.upstream_connect_timeout
-        if connect_timeout ~= null then
+        if connect_timeout then
           balancer_data.connect_timeout = connect_timeout
 
         else
@@ -561,7 +560,7 @@ return {
 
         local send_timeout = service.write_timeout or
                              api.upstream_send_timeout
-        if send_timeout ~= null then
+        if send_timeout then
           balancer_data.send_timeout = send_timeout
 
         else
@@ -570,7 +569,7 @@ return {
 
         local read_timeout = service.read_timeout or
                              api.upstream_read_timeout
-        if read_timeout ~= null then
+        if read_timeout then
           balancer_data.read_timeout = read_timeout
 
         else