This is required in situations where autoCommit.openSearcher is set to "false",
and you don't want to rely on core restarts to make committed index updates available to new searches.
It's already configured like this in the Common Web Platform environments,
so this change will mainly affect other environments (such as Silverstripe Platform).
To opt out of this new behaviour, set up a custom solrconfig.xml.
More details in https://github.com/silverstripe/silverstripe-fulltextsearch/issues/274
Solr::configure_server now takes "version" as one of the keys in the
option array, and behaves slightly differently depending on whether
that version is 3 or 4, to provide support for both Solr versions.
The Solr extras and templates have also moved, so that different
versions can be provided for the two different Solr versions.
