Improve stability and startup performance of SolrCloud with thousands of collections

A user on the mailing list with thousands of collections (5000 on 4.10.3, 4000 on 5.0) is having severe problems with getting Solr to restart.

I tried as hard as I could to duplicate the user setup, but I ran into many problems myself even before I was able to get 4000 collections created on a 5.0 example cloud setup. Restarting Solr takes a very long time, and it is not very stable once it's up and running.

This kind of setup is very much pushing the envelope on SolrCloud performance and scalability. It doesn't help that I'm running both Solr nodes on one machine (I started with 'bin/solr -e cloud') and that ZK is embedded.