use case insensitive linguistic sort and compare when connecting to oracle

was a cherry pick of https://github.com/owncloud/core/pull/8307 to master, forward port of https://github.com/owncloud/core/pull/8306

now switched to setting the the sort and compare session params when opening the connection.

cc @DeepDiver1975 @icewind1991 @PVince81 @karlitschek
Yes, it affects all comparison and sort operations: http://docs.oracle.com/cd/B19306_01/server.102/b14225/ch5lingsort.htm#CIHBFIID

Well this poses a problem when we consider usernames like `jorn`, `joern` and `jörn` which we currently treat as three distinct users.

I'll go back to a database specific SQL for the search query only, using REGEXP_LIKE for oracle and ILIKE for postgresql.