perf(db) yield on DB-less daos methods

### Summary

DBless doesn't naturally yield (not with `shared dict`, nor with `lmdb`). This may cause
latency spikes when iterating over bigger lists, e.g. `kong.db.routes:each()`. This commit
adds some yields so that iterating doesn't fully block the worker from doing other work
too, so this is about cooperative multitasking.

Author: bungle

kong/db/strategies/off/init.lua

@@ -2,7 +2,7 @@ local declarative_config = require "kong.db.schema.others.declarative_config"
 local workspaces = require "kong.workspaces"
 local lmdb = require("resty.lmdb")
 local marshaller = require("kong.db.declarative.marshaller")
-
+local yield = require("kong.tools.utils").yield
 
 
 local kong = kong
@@ -61,6 +61,8 @@ local function get_entity_ids_tagged(key, tag_names, tags_cond)
       return nil, err
     end
 
+    yield(true)
+
     list = list or {}
 
     if i > 1 and tags_cond == "and" then
@@ -141,6 +143,8 @@ local function page_for_key(self, key, size, offset, options)
     list = list or {}
   end
 
+  yield()
+
   local ret = {}
   local schema_name = self.schema.name
 
@@ -152,6 +156,8 @@ local function page_for_key(self, key, size, offset, options)
       break
     end
 
+    yield(true)
+
     -- Tags are stored in the cache entries "tags||@list" and "tags:<tagname>|@list"
     -- The contents of both of these entries is an array of strings
     -- Each of these strings has the form "<tag>|<entity_name>|<entity_id>"