Changed debug log to warn for when IW#rollback fails with an exception other than AlreadyClosedException
Add randomized plugin isolation to test infrastructure

fix #5296
Add docs for plugin isolation
[TEST] Fix SearchStatsTests to have all shards allocated

If randomization brings up a single shard per index in this test
we might run our searches on only one index which causes the assertions
to fail afterwards that's why we need to wait until everything is alloated.
[TEST] Beef up SearchStatsTests
[TEST] Make sure that a snapshot is completed before trying to modify repository