LPS-51413 As a system administrator I'd like to use 2-way SSL authentication against my SOLR server

Hi Mike,

This is that 2-way authentication feature for Solr we've talked about briefly at the DevCon.

* BasicAuthSolrServer was renamed to HttpSolrServer and the creation of HttpClient objects was delegated to two factory implementations, these encapsulate the details how BASIC or CERT authentication should be handled.
* I retained BasicAuthSolrServer, because it's been around for a while (2009) and wanted to maintain compatibility with the old configuration.
* Added usage examples, because setting up mutual SSL auth. tends to have some pitfalls (https://github.com/laszlocsontos/liferay-plugins/commit/9fb7de3280d583f7468a14bdbb07f69440ea3b71).
* Spot a race condition (https://github.com/laszlocsontos/liferay-plugins/commit/eafc4e9da96b32308f0e82b7906611a2bb1c35e7); threads serving requests might not be aware of the fact the Solr server has already been shut down, because they cannot see the current value of "_stopped".

OSGI is new to me and I'm not 100% sure how solr-blueprint.xml should have been modified, I can see some minor differences in bean definitions compared to solr-spring.xml, but I don't know their reason.

Please let me know if you want me to change something.

Thanks for reviewing!

Cheers,
László