Text to go in the release notes:
The configuration settings for specifying the memcached server is moved back to koha-conf.xml. Passing that information via an environment variable breaks badly on Plack.
It would allow command line scripts and cron jobs to also make use of caching. Not only plack.
ATTENTION on update: if you update from a config with memcached in koha-http.conf without Plack and you want to use Plack, you need to port memcached conf in koha-conf.xml by hand.
If you don't use Plack, you can leave memcached conf in koha-http.conf, but for the future is still better to migrate the conf by hand.