Selectively proxy to different backends to support older corpora

[jan-niestadt]

To allow users to still search older private corpora that are no longer supported by current BlackLab versions (because new Lucene versions drop support for older indexes), we could have the proxy forward requests to these older corpora to an older BlackLab instance, and all other requests to a modern instance of BlackLab. This would allow us to support these older corpora for a bit longer.

Support for the older corpora should be read-only, and we should warn users that support for these corpora will be dropped in a few years.

[jan-niestadt]

(the alternative, converting BlackLab corpora to new index formats, is certainly possible but probably more work)