Profiling fixes

A few fixes related to https://github.com/uwescience/myria-web/issues/226

(And one fix that isn't.)
Have you tested this? I wonder whether this index is more useful than one on `queryId` and `fragmentId`. I tried the indexes and at least postgres doesn't think there is a difference. 

```
 GroupAggregate  (cost=0.00..10479.85 rows=315 width=24) (actual time=0.019..0.019 rows=0 loops=1)
   ->  Index Scan using "public:logs__myria_indexes:Sending_0_1_4" on "public:logs:Sending"  (cost=0.00..10417.18 rows=5952 width=24) (actual time=0.018..0.018 rows=0 loops=1)
         Index Cond: ("queryId" = 4)
 Total runtime: 0.055 ms
```

```
 GroupAggregate  (cost=0.00..10479.85 rows=315 width=24)
   ->  Index Scan using "public:logs:Sending_queryId_fragmentId_destWorkerId_idx" on "public:logs:Sending"  (cost=0.00..10417.18 rows=5952 width=24)
         Index Cond: ("queryId" = 4)
```

```
 HashAggregate  (cost=10476.70..10479.85 rows=315 width=24) (actual time=0.022..0.022 rows=0 loops=1)
   ->  Index Scan using "public:logs:Sending_queryId_fragmentId_idx" on "public:logs:Sending"  (cost=0.00..10417.18 rows=5952 width=24) (actual time=0.021..0.021 rows=0 loops=1)
         Index Cond: ("queryId" = 4)
 Total runtime: 0.069 ms
```