Add the 'recovery' _cat API endpoint

This adds the _cat/recovery/{index} API endpoint, which displays information about the status of recovering replica shards. An example of the output:

```
$ curl 'localhost:9200/_cat/recovery'
index shard node                   target    recovered %
test2 0     Fwo7c_6MSdWM0uM1Ho4t-g 147304414 19236101  13.1%
test  0     Fwo7c_6MSdWM0uM1Ho4t-g 145891423 119640535 82.0%
```

Feedback welcome for the output format.

Fixes #3969