Open files with encoding='utf-8' in release script
Removed needless TestRiver used in RiverTests

DummyRiver can be used instead, no need for TestRiver
Add 0.90.10 to master
Expose min/max open file descriptors in Cluster Stats API

Also changes the response format of that section to:

```
 "open_file_descriptors": {
      "min": 200,
      "max": 346,
       "avg": 273
 }
```

Closes #4681

Note: this is an aggregate of 3 commits in the 0.90 branch
Cutover to Guava's Longs.compare.