[FTS] Ordering of search results when the specified sort field is not indexed is not consistent

Build : 4.7.0-1287

Test :
./testrunner -i fts.ini -t fts.stable_topology_fts.StableTopFTS.test_sorting_of_results_on_non_indexed_fields,items=100,sort_fields=salary,expected=emp10000042,emp10000001,emp10000071,cluster=D+F,F

Issue : It is understood that the field(s) used for sorting the search results MUST be indexed. However, if the field isnt indexed and is used for sorting, the ordering of search results are not consistent. If the same test is run multiple times, it doesn't return the same order each time. The above test is for a numeric field, but the same issue is observed in case of text and date_time fields as well.

Pls see the attached testrunner console output for more details. Test#1 & 2 are for numeric fields (same test repeated), #3 & 4 are for text field, #5 & 6 are for date_time field.