Add the 'recovery' _cat API endpoint

This addes the _cat/recovery/{index} API endpoint, which displays
information about the status of recovering shards. An example of the
output:

index shard node                   target    recovered     %
test2 0     Fwo7c_6MSdWM0uM1Ho4t-g 147304414  19236101 13.1%
test  0     Fwo7c_6MSdWM0uM1Ho4t-g 145891423 119640535 82.0%

Fixes #3969