Core: don't forcefully release IW lock (this can cause corruption)

This fixes the test failure at http://build-us-00.elasticsearch.org/job/es_core_1x_strong/1652/

I already fixed this in master with #8588 but I think we should back-port even though it's a behavior change: it's really dangerous to forcefully remove IW's lock on init (it can quickly cause index corruption as it did in this test failure, caught by our check index on close).

If the index is in fact locked it means somehow another ES instance is still open/writing to that store directory and it's better to get an exc saying that.
OK I'll fix these test cases to use a private store instead.