Document that SimpleSAML on PHP 7 should only use database session storage

[typhonius]

As discovered here: simplesamlphp/simplesamlphp#395 the SimpleSAMLphp library doesn't integrate well with memcache on PHP7.

The reason for this is that it assumes the existance of the Memcache class. Memcache is totally not ready for showtime on PHP7, whereas its sister Memcached is. Currently, Memcached is not supported by SimpleSAMLphp.

Acquia does not have the Memcache class, which causes fatal errors if memcache is selected as a session storage when using PHP > 7.

Are we best to default to database in the simplesaml template or keep the defaults and write some documentation on the simplesaml page to ensure unwitting teams don't get caught short?

[greylabel]

Just ran into this issue as well updating our ACE environments to PHP7 -- default to database in the simplesaml template seems reasonable.

[grasmash]

You could even detect PHP version and set accordingly. Otherwise, "default to database in the simplesaml template seems reasonable" to me too.

[grasmash]

@typhonius Would you mind submitting PR?