SERVER-13611 fix sort elimination in the case of trailing fields
(cherry picked from commit 9ec25e399fc775361ed37fd5abb78960659a308b)
SERVER-13496 do not abort replication on secondaries when index name conflicts (via 2.4.9)
(cherry picked from commit 0fbd76d233e213e43f53b8882c4dd3c71897a7f3)

Conflicts:
	src/mongo/base/error_codes.err
Merge remote-tracking branch 'upstream/v2.6' into v2.6