[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
Improve Settings#get lookup for camel case support
Today, if we miss on a get on setting, we try its camel case version. The assumption is that in our code, we never use camel case to lookup a setting, but we want to support camel case if the user provided one.
This can be expensive (#toCamelCase) when the get on the setting is done in a tight call, which is evident when running the allocation deciders as part of the reroute logic.
Instead of doing the camel case check on get, prepare an additional map that includes all the settings that are provided as came case, and try and lookup from it if needed.
closes #6765
Add v1.2.3 to Version.java
Fix typo in Hunspell logging
Docs: Update plugins.asciidoc

Closes #6683
Docs: New entry/front-end

Adding, Calaca, a simple search client for Elasticsearch.

Closes #6754
[Test] BackgroundIndexer uses the wrong logger

Closes #6781