Add Allocation Index Priority

[pickypg]

The idea is allow a simple, dynamically modifiable priority for indices. Something akin to:

$ curl -XPUT host:9200/my-index/_settings -d '{
  "index.priority" : 1
}'

By default, all priorities would default with index.priority : 0, thus being of equal priority unless modified. From there, the comparison could go to index creation date/time as the next priority level, then finally by name for the final order.

index.priority > index.creation_date > reverse sorted index name

This allows pretty much every use case to specify recovery order for best-case recovery order for their use case. Time-based solutions would pretty much work out of the box because the creation date would prioritize recovery of today's index first, thus giving the best chance to return to fully replicated indexing as soon as possible. Similarly, falling back to the reverse sorted index name gives arbitrary--yet predictable--sorting for simultaneously-created indices (however unlikely) that have no time basis (index-z, index-y, ...), but YMD-formatted date/time strings will properly sort (logstash-2015.05.03 will always come before logstash-2015.05.02 or earlier dates). Finally, the arbitrary priority allows users to take control of the process for cases where there is no time basis, such as when a "main" index is created, but then minor indices are created from time-to-time. Even better, this would be as simple as setting it once with a high value, then not setting or selectively setting the minor indices.

[pickypg]

This is a duplicate of #10069, but I had been thinking about it a bit over the course of the past few weeks and I wanted to write it down. We can close this one in favor of that one (or vice versa), but I wanted to get it written down.

[avleen]

I like the defaults proposed here :)

[s1monw]

FYI pushed the #11975 to 1.x and master