Make "cluster.routing.allocation.allow_rebalance" a dynamic setting

Also makes it a static constant and changes all tests to use it instead
of a string.

Fixes #7092
[TEST] increase ack timeout on large cluster
when we have a small machine running it with randmoized larger number of nodes (5), we need more time to process it
Resolve wording inconsistency

AND and OR filter docs talk about different targets for the operators. I
believe that both should be described in terms of modifying other 'filters'.
I also added articles for easier (human) parsing. This fixes #4762

Closes #7165
CORS: Allowed to configure allow-credentials header to work via SSL

This adds support to return the "Access-Control-Allow-Credentials" header
if needed, so CORS will work flawlessly with authenticated applications.

Closes #6380
Query Cache: Support shard level query response caching

The query cache allow to cache the (binary serialized) response of the shard level query phase execution based on the actual request as the key. The cache is fully coherent with the semantics of NRT, with a refresh (that actually ended up refreshing) causing previous cached entries on the relevant shard to be invalidated and eventually evicted.

This change enables query caching as an opt in index level setting, called `index.cache.query.enable` and defaults to `false`. The setting can be changed dynamically on an index. The cache is only enabled for search requests with search_type count.

The indices query cache is a node level query cache. The `indices.cache.query.size` controls what is the size (bytes wise) the cache will take, and defaults to `1%` of the heap. Note, this cache is very effective with small values in it already. There is also the advanced option to set `indices.cache.query.expire` that allow to control after a certain time of inaccessibility the cache will be evicted.

Note, the request takes the search "body" as is (bytes), and uses it as the key. This means same JSON but with different key order will constitute different cache entries.

This change includes basic stats (shard level, index/indices level, and node level) for the query cache, showing how much is used and eviction rates.

While this is a good first step, and the goal is to get it in, there are a few things that would be great additions to this work, but they can be done as additional pull requests:

- More stats, specifically cache hit and cache miss, per shard.
- Request level flag, defaults to "not set" (inheriting what the setting is).
- Allowing to change the cache size using the cluster update settings API
- Consider enabling the cache to query phase also when asking hits are involved, note, this will only include the "top docs", not the actual hits.
- See if there is a performant manner to solve the "out of order" of keys in the JSON case.
- Maybe introduce a filter element, that is outside of the request, that is checked, and if it matches all docs in a shard, will not be used as part of the key. This will help with time based indices and moving windows for shards that fall "inside" the window to be more effective caching wise.
- Add a more infra level support in search context that allows for any element to mark the search as non deterministic (on top of the support for "now"), and use it to not cache search responses.

closes #7161
Update API enhancement - add support for scripted upserts.
In the case of inserts the UpdateHelper class will now allow the script used to apply updates to run on the upsert doc provided by clients. This allows the logic for managing the internal state of the data item to be managed by the script and is not reliant on clients performing the initialisation of data structures managed by the script.

Closes #7143
[Query Cache] Add a request level flag to control query cache
A request level flag, defaults to be unset, to control the query cache. When not set, it defaults to the index level settings, when explicitly set, will override the index level setting
closes #7167
[TEST] make sure all shards have docs
we need that in order for refresh to be effective and actually refresh in the second round of indexing, otherwise, it caches a 0 docs shard and a refresh won't expire anything there
Fix false assert trip
Added an option to add arbitrary headers to the client requests

The headers are key/value pairs defined in the settings under the `request.headers` namespace.
[TEST] simplified unicast bw comp test

also fixed unchecked warning in CompositeTestCluster
Documented the query cache module

Related to #7161 and #7167
[TEST] Assert that put mappings requests are acked in GeoMappingTests.
[TEST] fixed unicast bw comp test configuration

The second internal node, when present, wasn't able to join the existing cluster due ti misconfigured unicast hosts, thus it would form its own cluster.
Internal: fixed typo in NodeService s/disovery/discovery
REST spec: Added missing query_cache param to clear_cache, nodes.stats and indices.stats

Relates to #7167 and #7161
Test: Trimmed trailing whitespace to make valid YAML
Tidied up the filters agg docs and added a coming[] tag
[DOCS] fixed title for filters aggregation documentation
[DOCS] Remove the section about codecs.

This documentation was dangerous because it felt like it was possible to gain
substantial performance by just switching the codec of the index.

However, non-default codecs are dangerous to use since they are not supported
in terms of backward compatibility, and most improvements that they bring have
been folded into the default codec anyway (for example, the default codec
"pulses" postings lists that contain a single document).