Java client API: CreateIndexRequestBuilder.addMapping throws IllegalStateException if you add same type more than once

Previously, it would silently overwrite the previous mapping, which was trappy.

Closes #7231

Closes #7243
Core: Store index creation time in index metadata

This change stores the index creation time in the index metadata when an index is created.  The creation time cannot be changed but can be set as part of the create index request to allow for correct creation times for historical data.

Closes #7119