Remove possibility for conflicting field definitions and ambiguous field resolution

[clintongormley]

Fields with the same name in different types in the same index should have the same mapping. Previously, this has been advised as "best practice", but relying on advice has proved insufficient (see #4081 for the many issues that have resulted from allowing conflicting field definitions). Instead we need to enforce this in the API.

This issue (which replaces #4081) is a meta issue listing all of the changes that need to be made:

• Move field mappings to the index level internally #8871 - Internally, field mappings should be stored at the index level, rather than at the type level. Types are essentially a way of grouping fields. While the mapping APIs will not change, an exception will be thrown if an attempt is made to map a field in one type in a way that conflicts with the mapping of a field with same name in another type. The _parent field is the one exception - this field will remain at the type level.
• Require the full path (without type) for field lookups #8872 Always require the full path for field names (wildcards still allowed), and remove the ability to prepend the full path with the _type.
• Remove type-level analyzer, index_analyzer, search_analyzer #8874 Remove type-level analyzer, search_analyzer, index_analyzer settings.
• Remove _analyzer #9279 Remove per-document _analyzer
• Mappings: disallow exotic options on meta fields #8143 Remove the ability to change the mapping for _uid, _id, _index, _type, _field_names.
• Deprecate extracting _routing and _id from document fields #6730 Deprecate extracting _routing and _id from document fields, but should still be possible to set _routing to required.
• Mappings: Ensure that reindexing is always possible #8142 Always store _timestamp, _ttl, _routing, _parent as these are required for reindexing.
• Mappings: Ensure that reindexing is always possible #8142 Should we also always require that the _source field should be stored? Disabling source prevents reindexing, updates, and out-of-the-box highlighting. Perhaps with the new compression options (Add best_compression option for indices #8863), the ability to disable the _source field is less useful?
• Remove the _boost field #8875 Remove the _boost field (deprecated in v1.0.0.RC1).
• Remove the ability to delete mappings #8877 Remove the ability to delete mappings.
• Mappings: Deprecate index_name and path #6677 Deprecate index_name and path settings
• Dealing with incompatible mapping changes #9443 Allow the user to upgrade mappings removing old settings without requiring reindexing (where possible)
• Throw an exception if field mapping is invalid #9927 Throw an exception if field mappings is invalid

Alternatives:

• Fields with different types can be renamed to distinguish their purpose, eg login_name vs login_date.
• Different types can be separated into different indices.