chore(deps) replace vendor mlcache with 2.0.1 upstream

This is part of a series of fixes:
- thibaultcha/lua-resty-mlcache#41
- thibaultcha/lua-resty-mlcache#42
- #3311
- #3341

Changes:

* remove vendor mlcache.lua file which had received patches for Kong
  support
* add lua-resty-mlcache 2.0.1 as a dependency
* implement custom IPC options for the DB mlcache instance

Changelog of mlcache 2.0.0:

  https://github.com/thibaultcha/lua-resty-mlcache/blob/master/CHANGELOG.md#200

Changelog of mlcache 2.0.1:

  https://github.com/thibaultcha/lua-resty-mlcache/blob/master/CHANGELOG.md#201

kong-0.13.0-0.rockspec

@@ -31,6 +31,7 @@ dependencies = {
   "lua-resty-worker-events == 0.3.2",
   "lua-resty-mediador == 0.1.2",
   "lua-resty-healthcheck == 0.4.0",
+  "lua-resty-mlcache == 2.0.1",
 }
 build = {
   type = "builtin",
@@ -47,8 +48,6 @@ build = {
 
     ["kong.cache"] = "kong/cache.lua",
 
-    ["kong.mlcache"] = "kong/mlcache.lua",
-
     ["kong.templates.nginx"] = "kong/templates/nginx.lua",
     ["kong.templates.nginx_kong"] = "kong/templates/nginx_kong.lua",
     ["kong.templates.kong_defaults"] = "kong/templates/kong_defaults.lua",

kong/cache.lua

@@ -1,4 +1,4 @@
-local kong_mlcache = require "kong.mlcache"
+local resty_mlcache = require "resty.mlcache"
 
 
 local type    = type
@@ -69,11 +69,23 @@ function _M.new(opts)
     return error("opts.resty_lock_opts must be a table")
   end
 
-  local mlcache, err = kong_mlcache.new(SHM_CACHE, opts.worker_events, {
+  local mlcache, err = resty_mlcache.new("kong_db_cache", SHM_CACHE, {
     lru_size         = LRU_SIZE,
     ttl              = max(opts.ttl     or 3600, 0),
     neg_ttl          = max(opts.neg_ttl or 300,  0),
     resty_lock_opts  = opts.resty_lock_opts,
+    ipc = {
+      register_listeners = function(events)
+        for _, event_t in pairs(events) do
+          opts.worker_events.register(function(data)
+            event_t.handler(data)
+          end, "mlcache", event_t.channel)
+        end
+      end,
+      broadcast = function(channel, data)
+        opts.worker_events.post("mlcache", channel, data)
+      end
+    }
   })
   if not mlcache then
     return nil, "failed to instantiate mlcache: " .. err
@@ -121,7 +133,7 @@ function _M:probe(key)
     return error("key must be a string")
   end
 
-  local ttl, err, v = self.mlcache:probe(key)
+  local ttl, err, v = self.mlcache:peek(key)
   if err then
     return nil, "failed to probe from node cache: " .. err
   end