Logs: Change log level for mpercolate

When using _mpercolate API we log by default a lot of DEBUG `Percolate shard response`.
They should be in TRACE level instead of DEBUG.

(cherry picked from commit 5bd720b)
Docs: Document action.replication_type setting

Document action.replication_type setting

Closes #8290
Use a 1024 byte minimum weight for filter cache entries

This changes the weighing function for the filter cache to use a
configurable minimum weight for each filter cached. This value defaults
to 1kb and can be configured with the
`indices.cache.filter.minimum_entry_weight` setting.

This also fixes an issue with the filter cache where the concurrency
level of the cache was exposed as a setting, but not used in cache
construction.

Relates to #8268
Update search-template.asciidoc

Fixed asciidoc typo

Closes #8308
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
Core: Allow to configure custom thread pools

Closes #8247