LPS-51413 Fix bug: empty password is actually blank, null doesn't work
LPS-51413 Implement certificate-based authentication
LPS-51413 Deprecate BasicAuthSolrServer and retain it for backward
compatibility
LPS-51413 Retain old spring config for backward compatibility
LPS-51413 Expose new API to Spring and OSGi with usage examples
LPS-51413 Add logging configuration
LPS-51413 Fix race condition - not related

It's unlikely that "shutdown" would ever be called twice, however it's
possible for "request" to be called from several simultaneous threads at
the same time shutdown is invoked. If "_stopped" is just an ordinary
field, threads serving requests might not see its current value and
therefore not be aware that the Solr server has already been shut down.