[Engine] checkVersionMapRefresh shouldn't use indexWriter.getConfig(). We run it out of lock, the indexWriter may be closed..

Relates to #6443, #6786

Closes #6794
Update preference.asciidoc

Clarify that `preference` is a query string parameter only
and provide an example.
[Code] Control whether MapperService docMapper iterator should contain DEFAULT_MAPPING

At the moment one can iterate the MapperService to go through all document mappers. This includes the document mapper of DEFAULT_MAPPING, which may be surprising and lead to unintended results. This commit removes the Iterable implementation and add a docMappers method that asks the caller to make an explicit choice

Closes #6793
Store: Before removing shard physically from disk verify that another node in the cluster actually holds an active shard copy.

Closes #6692