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.
Test: dump all threads when delete index fails during test cleanup
Core: still don't load bloom filters, even when Directory instance doesn't have a codecService
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
Rest API: indices.delete should have "index" as a required param
[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
Logging: Add log4j-extras dependency

Close #7927
[DOCS] Added documentation for log4j-extras dependency
Update migrate_1_4.asciidoc

Added note about MVEL being removed to breaking changes.

Closes #8462
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
Provide more context variables in update scripts

In addition to `_source`, the following variables are available through
the `ctx` map: `_index`, `_type`, `_id`, `_version`, `_routing`,
`_parent`, `_timestamp`, `_ttl`.

Some of these fields are more useful still within the context of an
Update By Query, see #1607, #2230, #2231.
Internal: Don't pass acceptdocs down to nonNestedDocsFilter in ParentsFilter

Closes #8461
Mapping: Fixes Merging of default analyzer

Fixed behaviour where two representations of the default index analyzer weren't being treated as equivalent. Added REST test to confirm fix.

Closes #2716
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