Support for ElastiCache Autodiscovery

[nikolay]

http://docs.aws.amazon.com/AmazonElastiCache/latest/UserGuide/AutoDiscovery.html

[agentzh]

@nikolay I think it's much easier to do on the Lua land via the ngx_lua module atop the lua-resty-memcached library. It would be too hard to build this into this ngx_memc module I'm afraid.

[nikolay]

@agentzh Is there a way to implement the inventory autodiscovery in Lua, but keep the rest in this module?

[agentzh]

@nikolay That's possible with rewrite_by_lua* or alike but less efficient and less flexible.