[TEST] Enforce provided settings in test base classes

Enviorment variables might override the tests settings even if
they are explicitly set. Other base classes like InternalTestCluster
also specify `config.ignore_system_properties: true` to ensure `what
we set is what we get`
Test: make test less evil so it doesn't take so long
[TEST] Enforce that the node from ElasticsearchSingleNodeTest is alone in its cluster.

In particular, this removes the ability to create new node and adds the ability
to get a reference to the single node that the cluster is made of.
Docs: explain that SerialMergeScheduler just maps to CMS for back compat

Closes #6878
Mappings: Add transform to document before index.

Closes #6566
[FIX] normalize serialization of ScriptType in UpdateRequest
Fix transient testScore failure by making DF consistent for query.
Share numeric data analyzer instances between mappings
use similar mechanism that shares numeric analyzers for long/double/... for dates as well. This has nice memory save properties with many date fields mapping case, as well as analysis saves (thread local resources)
closes #6843
Releasable XContentBuilder
make the builder releasable (auto closeable), and use it in shards state
also make XContentParser releasable (AutoCloseable) and not closeable since it doesn't throw an IOException
closes #6869
[AGGS] Pass current docid being processed to scripts.

Scripts may internally cache based on docid (as expressions do). This
change makes numeric aggregations using scripts pass the docid when
it changes.
Remove setNextScore in SearchScript.

While it would be nice to do this all the way up the chain (into
score functions), this at least removes the weird dual
setNextScore/setScorer for SearchScripts.