kv: test that verifies we don't emit duplicate intents
kv: test that verifies we don't emit duplicate intents

In #5042 we had an issue with transactions that wrote a key and then wrote the
same key as part of the final batch (with commit). This change adds a test that
verifies that the TxnCoordSender does not generate duplicate intents and that it
merges intents for overlapping ranges.

glycerine/cockroach,glycerine/cockroach,bg451/cockroach,nieyy/cockroach,sunya123/cockroach,bg451/cockroach,sunya123/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,petermattis/cockroach,maxlang/cockroach,arjun4084346/cockroach,glycerine/cockroach,nieyy/cockroach,sunya123/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,glycerine/cockroach,paperstreet/cockroach,petermattis/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,petermattis/cockroach,glycerine/cockroach,arjun4084346/cockroach,petermattis/cockroach,petermattis/cockroach,paperstreet/cockroach,maxlang/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,nieyy/cockroach,nieyy/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,sunya123/cockroach,veteranlu/cockroach,nieyy/cockroach,bg451/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,maxlang/cockroach,paperstreet/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,glycerine/cockroach,maxlang/cockroach,vaibhavarora/cockroach,petermattis/cockroach,glycerine/cockroach,sunya123/cockroach,paperstreet/cockroach,arjun4084346/cockroach,nieyy/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,maxlang/cockroach,paperstreet/cockroach,maxlang/cockroach,veteranlu/cockroach,veteranlu/cockroach,glycerine/cockroach,bg451/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,maxlang/cockroach,sunya123/cockroach,vivekmenezes/cockroach,nieyy/cockroach,paperstreet/cockroach,cockroachdb/cockroach,veteranlu/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,veteranlu/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,petermattis/cockroach,sunya123/cockroach,bg451/cockroach,petermattis/cockroach,sunya123/cockroach,arjun4084346/cockroach,maxlang/cockroach,bg451/cockroach,nieyy/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,bg451/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach