Aggregations: Buckets can now be serialized outside of an Aggregation

This change means that buckets can now be serialised to JSON and serialized and deserialized to the transport API outside of the aggregation that contains them.  This is a required change for #8110 (Reducers framework) but should make sense on it's own since object should really take care of their own serialization rather than relying on their parent object.
Internal: use 1 instead of 0 as filler version value for nested docs
Netty: Support to bind on multiple host/port pairs

This patch allows to create several netty bootstrap, each of which
listening on different ports. This will potentially allow for features
to listen to different network interfaces for node-to-node or node-to-client
communication and is also the base to listen to several interfaces, so that those
can be used to speed up cluster communication in the future.

Closes #8098
Test: Fix netty multiport tests

Ensuring, that even in local mode, netty is started appropriately.
Core: fix index throttling to notice a change to max_merge_count on the next merge start/finish

Previously such changes were never noticed, and index throttling kept
kicking in at the original setting from startup.

Closes #8136

Closes #8132
Core: cutover to Lucene's query rescorer

This is functionally equivalent to before, so there should be no
user-visible impact, except I added a NOTE in the docs warning about
the interaction of pagination and rescoring.

Closes #6232

Closes #7707
Test: ignore this for now
Test: ignore this for now
Tests: re-enable these tests
Docs: Added 'd' to the list of supported units.

Day was missing from the list of supported units in the date math section.

Closes #8151
[Analysis] Parse synonyms with the same analysis chain

Close #7199
[Analysis] Parse synonyms with the same analysis chain
 Fix backword compat logic and add test

Close #7199