Summary:
10 seems better at no visible CPU cost. Not very scientific as this is
only one data point, but neither was choosing 5 in the first place.
Measurements on OpenSSL using Pulse.ISL:
```
$ time infer --pulse-only --scheduler callgraph -j 2 --pulse-report-latent-issues --pulse-isl
| fuel | user time (s) | under-normalisation | latent issues |
|------+---------------+---------------------+---------------|
| 5 | 163 | 3074 | 160 |
| 10 | 158 | 85 | 160 |
| 15 | 174 | 32 | 160 |
| 20 | 186 | 20 | 160 |
```
Reviewed By: skcho
Differential Revision: D27156497
fbshipit-source-id: 1114b8677