…found" with an out-dated ClusterState (apache#2363)"

This reverts commit 5c399dd.

This commit augments our embedded-ZK code to support running embedded-ZK
in "quorum" or ensemble mode.  Multiple Solr nodes can now all have
their embedded-ZK's join a multi-node quorum upon startup.  Other than
Solr and ZK sharing a process, the embedded- ZK ensemble behaves
identically to one formed of independent processes: nodes can join or
leave the cluster, etc.

Embedded-ensemble-ZK is enabled any time the `zkQuorumRun` system
property is present, along with an explicitly specified ZK host string.
On startup, Solr will identify which host in the zk-conn-string it
should be (based on admittedly hacky heuristics), and then spins up a
'ZooKeeperServerEmbedded' instance in-process to join the ensemble. e.g.

```
export LH="localhost"
bin/solr start -p 8983 -z $LH:9983,$LH:9984,$LH:9985 -DzkQuorumRun
bin/solr start -p 8984 -z $LH:9983,$LH:9984,$LH:9985 -DzkQuorumRun
bin/solr start -p 8985 -z $LH:9983,$LH:9984,$LH:9985 -DzkQuorumRun
```

Some notes:
  - this doesn't (yet) work with ZK's dynamic-ensemble feature, so all
    ZK nodes must be specified in a static ZK conn string provided at
    startup
  - this appears to run best when the security-manager is disabled.

…re embedded in Solr