Pull replication appears to consume excessive CPU as it does not use Segment Reader pooling

While we are experimenting with adding PULL replica to our solr cluster, it's found from profiling that IndexFetcher seems to impose much more CPU overhead than anticipated , and most of those CPU time are spent in `SegmentReader.init` which is called by `SolrCore.openNewSearch` from the `IndexFetcher`.

With some debugging, it's found that for every replication on an updated collection, a new `SegmentReader` is created for every segment for such collection (not only the ones that are pulled down), compared to `SolrCore.openNewSearch` triggered from regular commit, which ONLY creates `SegmentReader` for new segments, which old `SegmentReader`s are obtained from `ReaderPool`.

Unfortunately, such pool does not work for `IndexFetcher` as it opens a new `IndexWriter` on every run at here, which creates a new `ReaderPool`.

I am not familiar enough to tell whether such new `IndexWriter` is always needed, but opening `SegmentReader` for every segment of a collection seems excessive for pull replication.

Any thoughts on this please? Many thanks!!