[6.x.x] Adds in memory cache for RPC query results

[reinhapa]

Backport of https://github.com/eXist-db/exist/pull/5188.

Enables in-memory query result values less than 4 MB instead of writing them to a temporary file.

By using the in-memory-buffer no potential virus scanner is trigger either on the server as on the client side. This almost doubled the performance on our productive systems, when using this change.

[line-o]

Uhh, I need to see what this does for xst

[line-o]

@reinhapa should this be forward ported to existdb 7 or is it already in place there?

[adamretter]

should this be forward ported to existdb 7 or is it already in place there?

Review is already happening at the source for eXist-db 7.x.x - https://github.com/eXist-db/exist/pull/5188

[reinhapa]

@adamretter PR is ready for review 😊