Allow on demand analyses to be cancelled

Add a mechanism to cancel an analysis once started. This would have the benefit of making the list of analyses clearer.

Request from Mike Drob (Apache Solr) https://sonatypelift.slack.com/archives/C02MG23D13P/p1649121659760599

I started an analysis and then fixed a bug about five minutes later and wanted to start a new analysis. The results of the first one wouldn’t matter anymore.