mappings: keep parameters in mapping for _timestamp and _size even if di...

...sabled

Settings that are not default for _size and _timestamp were only build in
toXContent if these fields were actually enabled.
_timestamp 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 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.
Mappings are modified in place, so potentially the index could be removed between the call to hasIndex and the call to indexServiceSafe.