Enable memcached and use build stages for smaller image

[ziouf]

enable memcached integration by

• installing libmemcached dependencies
• enabling in ./configure run

use build stages for smaller final image

$ docker image ls
REPOSITORY      TAG               IMAGE ID       CREATED        SIZE
pgpool/pgpool   4.4.3-memcached   b324557118c0   15 hours ago   63.1MB  <-- new image size
pgpool/pgpool   4.4.3             1db5b775ea5a   3 weeks ago    709MB   <-- from docker hub

@pengbo0328 @pgpool