Make allocation deciders more debuggable

[clintongormley]

Currently, the excellent _cluster/reroute?explain API gives very detailed information about why a particular shard could or couldn't be moved, but we have little insight into why a cluster doesn't rebalance shards automatically.

eg if you change a setting like cluster.routing.allocation.balance.primary, and nothing changes, it is difficult to know why nothing has changed.

It would be very useful to have more insight into the decisions made by each allocation decider for each shard.

[nik9000]

Sometimes I find that the allocation decider has to be "kicked" by manually forcing a move. I'm not sure what the deal is there but something like this could help debug that.

[clintongormley]

@nik9000 the allocation deciders aren't checked periodically, but only when something changes. We've opened #8146 for that one.

[nik9000]

@clintongormley thanks! It probably is that, yeah.

[clintongormley]

Closing in favour of #12412