mappings: keep parameters in mapping for _timestamp, _index and _size even if disabled

Settings that are not default for _size, _index and _timestamp were only build in
toXContent if these fields were actually enabled.
_timestamp, _index and _size can be dynamically enabled or disabled.
Therfore the settings must be kept, even if the field is disabled.
(Dynamic enabling/disabling was intended, see TimestampFieldMapper.merge(..)
and SizeMappingTests#testThatDisablingWorksWhenMerging
but actually never worked, see below).

To avoid that _timestamp is overwritten by a default mapping
this commit also adds a check to mapping merging if the type is already
in the mapping. In this case the default is not applied anymore.
(see
SimpleTimestampTests#testThatUpdatingMappingShouldNotRemoveTimestampConfiguration)

As a side effect, this fixes
- overwriting of paramters from the _source field by default mappings
  (see DefaultSourceMappingTests).
- dynamic enabling and disabling of _timestamp and _size ()
  (see SimpleTimestampTests#testThatTimestampCanBeSwitchedOnAndOff and
  SizeMappingIntegrationTests#testThatTimestampCanBeSwitchedOnAndOff )

Tests:

Enable UpdateMappingOnClusterTests#test_doc_valuesInvalidMappingOnUpdate again
The missing settings in the mapping for _timestamp, _index and _size caused a the
failure: When creating a mapping which has settings other than default and the
field disabled, still empty field mappings were built from the type mappers.
When creating such a mapping, the mapping source on master and the rest of the cluster
can be out of sync for some time:

1. Master creates the index with source _timestamp:{_store:true}
   mapper classes are in a correct state but source is _timestamp:{}
2. Nodes update mapping and refresh source which then completely misses _timestamp
3. After a while source is refreshed again also on master and the _timestamp:{}
   vanishes there also.

The test UpdateMappingOnCusterTests#test_doc_valuesInvalidMappingOnUpdate failed
because the cluster state was sampled from master between 1. and 3. because the
randomized testing injected a default mapping with disabled _size and _timestamp
fields that have settings which are not default.

The test
TimestampMappingTests#testThatDisablingFieldMapperDoesNotReturnAnyUselessInfo
must be removed because it actualy expected the timestamp to remove
parameters when it was disabled.

closes #7137