Update search-template.asciidoc

Fixed asciidoc typo

Closes #8308
Docs: Add elastic client for Google Go.

Add elastic, an Elasticsearch client for Google Go.

Closes #8302
Update search-template.asciidoc

Clarified using the conditional clause template example as a string
Netty: Add HTTP pipelining support

This adds HTTP pipelining support to netty. Previously pipelining was not
supported due to the asynchronous nature of elasticsearch. The first request
that was returned by Elasticsearch, was returned as first response,
regardless of the correct order.

The solution to this problem is to add a handler to the netty pipeline
that maintains an ordered list and thus orders the responses before
returning them to the client. This means, we will always have some state
on the server side and also requires some memory in order to keep the
responses there.

Pipelining is enabled by default, but can be configured by setting the
http.pipelining property to true|false. In addition the maximum size of
the event queue can be configured.

The initial netty handler is copied from this repo
https://github.com/typesafehub/netty-http-pipelining

Closes #2665
Search: Reduce memory usage during fetch source sub phase.

If includes or excludes are set
XContentFactory.xcontentBuilder() allocates a new
BytesStreamOutput using the default page size which is 16kb.

Can be optimized to use the length of the sourceRef because
that is the maximum possible size that the streamOutput will
use.

This redcues the amount of memory allocated for a request
that is fetching 200.000 small documents (~150 bytes each)
by about 300 MB

Close #8138
Tests: Move logSegmentsState to shared location, and remove no longer
needed verbose logging from upgrade test.
Core: Allow to configure custom thread pools

Closes #8247