Add correct Content-Length on HEAD requests

This commit fixes responses to HEAD requests so that the value of the
Content-Length is correct per the HTTP spec. Namely, the value of this
header should be equal to the Content-Length if the request were not a
HEAD request.

This commit also fixes a memory leak on HEAD requests to the main action
that arose from the bytes on a builder not being released due to them
being dropped on the floor to ensure that the response to the main
action did not have a body.

Relates elastic#21123

Updated release notes for 5.0.0-rc1

Added 2.4.1 release notes

Convert more docs to CONSOLE

* reference/api-conventions.asciidoc
* reference/redirects.asciidoc

Relates to elastic#18160

AllTermQuery must implement equals/hashCode. elastic#20196

This is required as queries are used as keys in the filter cache. Currently
all AllTermQuery instances are considered equals.

Recovery: don't log an error when listing an empty folder

[DOCS] fix documentation for selecting algorithm for percentiles agg

[doc] explain avg in function_score better (elastic#19154)

* [doc] explain avg in function_score better

Randomize packaging upgrade test

This commit adds randomization for the packaging upgrade test. In
particular, we extract a list of the released version of Elasticsearch
from Maven Central and randomize the selection of the version to upgrade
from. The randomization is repeatable, and supports the tests.seed
property. Specific versions can be tested by setting the property
tests.packaging.upgrade.from.versions.

Relates elastic#19033

Validate parameters of native sig score scripts so that we know which…

… ones are not set.