Ryan Ernst b96bd201c1 Mappings: Lock down _id field
There are two implications to this change.
First, percolator now uses _uid internally, extracting the id portion
when needed. Second, sorting on _id is no longer possible, since you
can no longer index _id. However, _uid can still be used to sort, and
is better anyways as indexing _id just to make it available to
fielddata for sorting is wasteful.

see #8143
closes #9842
2015-02-24 14:26:22 -08:00
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