Core: Remove NestedDocsFilter, because it isn't used and also don't eagerly load it in bitset filter cache.

Closes #8414
[docs] add 2d vis for decay functions and parameters

closes #8420
Scripting: Add score() back to AbstractSearchScript

See #8377
closes #8416
closes #8417
Rest API: indices.delete should have "index" as a required param
Docs: Fix incorrect documentation for the `index.query.parse.allow_unmapped_fields` setting.

The `index.query.parse.allow_unmapped_fields` setting can't influence whether unmapped fields are allowed
in alias filters and percolator queries.
Docs: note about confusing disk threshold settings
Discovery: Removed unnecessary DiscoveryService reference from LocalDiscover, which causes circular Guice dependencies errors.
Updated threadpool documentation to elaborate/clarify what the pools are for and their values

Closes #8446
[DOCS] Corrected syntax error in search curl cmd

Closes #8447
[DOCS] Expand logging documentation

Updated log4j link so it doesn't point to log4j 2.0 but version 1.2. Clarified which formats are supported and briefly explained what loggers and appenders are, plus added a link to the log4j docs.

Closes #5305
Closes #8455
Update migrate_1_4.asciidoc

Added note about MVEL being removed to breaking changes.

Closes #8462
Reroute shards automatically when high disk watermark is exceeded

This adds a Listener interface to the ClusterInfoService, this is used
by the DiskThresholdDecider, which adds a listener to check for nodes
passing the high watermark. If a node is past the high watermark an
empty reroute is issued so shards can be reallocated if desired.

A reroute will only be issued once every
`cluster.routing.allocation.disk.reroute_interval`, which is "60s" by
default.

Refactors InternalClusterInfoService to delegate the nodes stats and
indices stats gathering into separate methods so they have be overriden
by extending classes. Each stat gathering method returns a
CountDownLatch that can be used to wait until processing for that part
is successful before calling the listeners.

Fixes #8146
Take percentage watermarks into account for reroute listener

Fixes an issue where only absolute bytes were taken into account when
kicking off an automatic reroute due to disk usage. Also randomized the
tests to use either an absolute value or a percentage so this is tested.

Also adds logging for each node over the high and low watermark every
time a new cluster info usage is gathered (defaults to every 30
seconds).

Related to #8368
Fixes #8367
Indices API: Fix GET index API always running all features

Previous to this change all features (_alias,_mapping,_settings,_warmer) are run regardless of which features are actually requested. This change fixes the request object to resolve this bug
Added quotes to allow spaces in installation path
Tests: Improve netty test behaviour

Based on some test failures, this commit fixes two minor things

* Bind ports only on so called ephemeral ports to prevent try to
  bind to ports where elasticsearch already runs on
* Remove @Network annotation as it was used in a wrong scope
Update terms-aggregation.asciidoc

It looks like the reference editor forgot to remove some draft notes. These two sentences look more like a release notes than the reference info.

Closes #8469
Internal: Don't pass acceptdocs down to nonNestedDocsFilter in ParentsFilter

Closes #8461
Configuration: Tab characters in YAML should throw an exception.

Throw an exception if there is a 'tab' character in the elasticsearch.yml file

Close #8259
Core: In the bitset cache only eagerly load bitsets for parent nested object fields.
Don't eagerly cache parent type filters in bitset cache or nested object fields that are leafs.

Also let parent/child queries not rely on FixedBitSetFilter, but rather on regular Filter

Closes #8440