Improve indexing speed when using Token Authentication

Update: Token Authentication doesn’t seem to be to blame after all. Instead, it seems people are hitting SearchStax’s relatively low request size limit, which activates a fallback mechanism in the Solr backend plugin which sends the documents one at a time instead.

While affected users can just reduce their configured indexing batch size to work around this problem, I’ll investigate whether we can also provide a fix in the module code.

Implemented in this MR.

Merged, will test with the dev snapshot.