Fix mapping creation on bulk request

When a bulk request triggers an index creation a mapping might not be
created. The reason is that when indexing documents in a bulk,
an indexing operation might fail due to a shard not yet being
started. The mapping service, however, might already
have the mapping but the mapping update is never issued to the master,
even on subsequent indexing of documents.

Instead, the mapping must be propagated to master even if the
indexing fails due to a shard not being started.

closes #5623
Remove ScriptDocValues.EMPTY.

Instead the default implementation is used, but on top of empty
(Bytes|Long|Double|GeoPoint)Values. This makes sure there is no
inconsistency between documents depending on whether other documents in the
segment have values or not.

Close #5646
[TEST] Use NoMergePolicy rather than NoMergeScheduler in FreqTermsEnumTests
Document es.node.mode and add possible options to error message
Add `flush` method for BulkProcessor class

There is no explicit method `flush/execute` in `BulkProcessor` class. This can be useful in certain scenarios.
Currently it requires to close and create a new BulkProcessor if one wants an immediate flush.

Closes #5575.
Closes #5570.