[Engine] `index.fail_on_corruption` is not updateable

The `index.fail_on_corruption` was not updateable via the index settings
API. This commit also fixed the setting prefix to be consistent with other
setting on the engine. Yet, this feature is unreleased so this won't break anything.

Closes #6941
[RESTORE] Fail restore if snapshot is corrupted

today if a snapshot is corrupted the restore operation
never terminates. Yet, if the snapshot is corrupted there
is no way to restore it anyway. If such a snapshot is restored
today the only way to cancle it is to delete the entire index which
might cause dataloss. This commit also fixes an issue in InternalEngine
where a deadlock can occur if a corruption is detected during flush
since the InternalEngine#snapshotIndex aqcuires a topLevel read lock
which prevents closing the engine.

Closes #6938
[TEST] Activate test in PluginManagerTests that is supposed to be fixed
[TEST] Stress test for update and delete concurrency.

This test deletes and updates using upserts documents over several threads in a
tight loop. It counts the number of responses and verifies that the versions at
the end are correct.
Merge branch 'master' of github.com:elasticsearch/elasticsearch
[TEST] Plugin Manager

Undo removal of change.
Merge branch 'master' of github.com:elasticsearch/elasticsearch
[TEST] Remove TransportUpdateActionTest

This test has been made obselete by the UpdateTests.
Docs: Typo
typo

causes the example to fail in bash