Shard "state" flag is confusing and of limited value to outside consumers

Solr API (and consequently the metric reporters, which are often used for Solr monitoring) report the shard as being in ACTIVE state even when in reality its functionality is severely compromised (eg. no replicas, all replicas down, or no leader).

This reported state is technically correct because it is used only for tracking of the SPLITSHARD operations, as defined in Slice.State. However, this may be misleading and more often unhelpful than not - for constant monitoring a flag that actually reports impaired functionality of a shard would be more useful than a flag that reports a relatively uncommon SPLITSHARD operation.

We could either redefine the meaning of the existing flag (and change its state according to some of the criteria I listed above), or add another flag to represent the "health" status of a shard. The value of this flag would then provide an easy way to monitor and to alert external systems of dangerous function impairment, without monitoring the state of all replicas of a collection.