force blind puts into non-empty key range

The insert workload is randomized, so the blind puts were computed but nothing
was actually written blindly.

benchmark                                 old ns/op     new ns/op     delta
BenchmarkTrackChoices1280_Cockroach-8     171532        65505         -61.81%

benchmark                                 old allocs     new allocs     delta
BenchmarkTrackChoices1280_Cockroach-8     142            130            -8.45%

benchmark                                 old bytes     new bytes     delta
BenchmarkTrackChoices1280_Cockroach-8     27241         26018         -4.49%

```
2016/05/10 20:15:44.886244	0.006944	node 1
20:15:44.886246	 .     3	... node 1
20:15:44.886253	 .     6	... read has no clock uncertainty
20:15:44.886334	 .    81	... executing 1282 requests
20:15:44.886543	 .   209	... read-write path
20:15:44.886544	 .     1	... command queue
20:15:44.886963	 .   420	... left command queue
20:15:44.886966	 .     3	... request leader lease (attempt cockroachdb#1)
20:15:44.887265	 .   298	... prep for ts cache
20:15:44.887266	 .     1	... applied ts cache
20:15:44.887268	 .     2	... start marshal
20:15:44.887772	 .   504	... end marshal
20:15:44.887835	 .    63	... begin prop
20:15:44.887852	 .    17	... done prop
20:15:44.887855	 .     3	... proposed to Raft
20:15:44.888949	 .  1094	... applying batch
20:15:44.889015	 .    67	... checked aborted txn
20:15:44.889197	 .   181	... checked leadership
20:15:44.889199	 .     2	... executing as 1PC txn
20:15:44.889644	 .   445	... 1280 blind puts
20:15:44.892245	 .  2601	... executeBatch returns
20:15:44.892308	 .    63	... executed batch
20:15:44.892360	 .    52	... prep for commit
20:15:44.893101	 .   741	... committed
20:15:44.893154	 .    53	... processed async intents
20:15:44.893155	 .     1	... applied batch
20:15:44.893173	 .    18	... response obtained
20:15:44.893174	 .     1	... endCmds begins
20:15:44.893175	 .     1	... begin update tsCache
20:15:44.893176	 .     1	... end update tsCache
20:15:44.893177	 .     2	... removed from cmdQ
20:15:44.893178	 .     1	... endCmds ends
20:15:44.893180	 .     1	... Send returns
```

out.allblind.10s

@@ -0,0 +1,3 @@
+PASS
+BenchmarkTrackChoices1280_Cockroach-8	  200000	     65505 ns/op	   26018 B/op	     130 allocs/op
+ok  	github.com/cockroachdb/cockroach/sql	14.236s

out.allblind.1s

@@ -0,0 +1,3 @@
+PASS
+BenchmarkTrackChoices1280_Cockroach-8	   20000	     63429 ns/op	   26003 B/op	     130 allocs/op
+ok  	github.com/cockroachdb/cockroach/sql	2.359s

out.allblind.20s

@@ -0,0 +1,3 @@
+PASS
+BenchmarkTrackChoices1280_Cockroach-8	  500000	     79451 ns/op	   29265 B/op	     145 allocs/op
+ok  	github.com/cockroachdb/cockroach/sql	40.904s

storage/replica.go

@@ -1994,7 +1994,7 @@ func optimizePuts(ctx context.Context, batch engine.Engine, reqs []roachpb.Reque
 	iter.Seek(engine.MakeMVCCMetadataKey(minKey))
 	var iterKey roachpb.Key
 	if iter.Valid() && bytes.Compare(iter.Key().Key, maxKey) <= 0 {
-		iterKey = iter.Key().Key
+		// iterKey = iter.Key().Key
 	}
 	// Set the prefix of the run which is being written to virgin
 	// keyspace to "blindly" put values.