Open files with encoding='utf-8' in release script
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
Added stream serialization logic for new min/max file descriptors

Relates to #4681
Backported missing RiverTests
Changed ClusterStatsNodes.ProcessStats stream serialization logic to use readLong instead of readVLong for file descriptors

The minOpenFileDescriptors/maxOpenFileDescriptors can be negative valued if the ClusterStatsNodes response was initialized with no nodes.

Relates to #4681
release [0.90.10]
Move to [0.90.11] snap