Improve handling of store parameter updates post indexing

[ppf2]

DELETE /myindex
POST /myindex/mytype/1
{
  "prop1":"test1"
}
GET /myindex/_search
{
  "fields": [
    "_timestamp"
  ], 
  "query": {
    "match_all": {}
  }
}
PUT /myindex/mytype/_mapping
{

  "mytype": {
    "_timestamp": {
    "enabled": "true",
    "store": "true"
    }
  }
}
GET /myindex/mytype/_mapping

If you create an index, put a doc into it, and then try to update the mapping to enable _timestamp and set store:true, you will notice that the response comes back as acknowledged:true with no error messages. But if you get the _mapping back, you will notice that store:true is not set (by design because we do not allow changing the store parameter post indexing).

For regular properties, if you attempt to update its store parameter post indexing, it will actually throw a 400 error indicating a MergeMappingException because of differences in store values.

It will be nice to add the same validation and throw a similar error when users attempt to update the store value of _timestamp post-indexing.

[brwe]

Seems to the the same issue as #777