Explore using a engine-private IndexReader for realtime-get etc. #26912
Description
This is a spinoff from #26802 where we potentially pay a big price on rebuilding caches etc. when updates / realtime-get is used. We are refreshing the internal reader for realtime gets which causes for instance global ords to be rebuild. I think we can gain quite a bit if we'd use a reader that is only used for GETs and not for searches etc. that way we can also solve problems of searchers being refreshed unexpectedly aside of replica recovery / relocation.