Mapping: Fix delete mapping race condition.

If multiple clients attempted to delete a mapping, or a single client attempted to delete a mapping as an index is being
created a NPE could be observed in the MetaDataMappingService. This fix makes sure we don't try to access a null indexMetaData object.
Also add a test to spawn multiple create and delete threads to provoke this race condition.

Closes #5997
`assertAcked` here too