* Expose index age in ILM explain output
This adds the index's age to the ILM explain output, for example:
```
{
"indices" : {
"ilm-000001" : {
"index" : "ilm-000001",
"managed" : true,
"policy" : "full-lifecycle",
"lifecycle_date" : "2019-07-16T19:48:22.294Z",
"lifecycle_date_millis" : 1563306502294,
"age" : "1.34m",
"phase" : "hot",
"phase_time" : "2019-07-16T19:48:22.487Z",
... etc ...
}
}
}
```
This age can be used to tell when ILM will transition the index to the
next phase, based on that phase's `min_age`.
Resolves#38988
* Expose age in getters and in HLRC
fe2ef66e45