memcached: rename internal module function memcached_init()

Name of internal function for creating a new memcached instance is
memcached_init(). It may be confusing - name can be mixed up with
name of function for module initialization.

In the following commit we will need an explicit function
for module initialization, memcached_init() was renamed to
memcached_create_instance() for clarity.

This commit changes internal function's name, no impact on public API.

Needed for #96

memcached/init.lua

@@ -332,7 +332,7 @@ local memcached_methods = {
     end
 }
 
-local function memcached_init(name, uri, opts)
+local function memcached_create_instance(name, uri, opts)
     opts = opts or {}
     if memcached_services[name] ~= nil then
         if not opts.if_not_exists then
@@ -387,7 +387,7 @@ local function memcached_get(name)
 end
 
 return {
-    create = memcached_init,
+    create = memcached_create_instance,
     get    = memcached_get,
     server = setmetatable({}, {
         __index = memcached_services