Fetching backlinks is slow and consumes a lot of memory on large instances

Steps to reproduce:

1. Create a lot of pages, or pages with lots of objects/object properties. 37 million indexed items (pages + objects + object properties + attachments) should make the problem noticeable.
2. View the backlinks of a document that has few backlinks.

Expected result:

Viewing the backlinks is fast.

Actual result:

Viewing the backlinks is relatively slow, requesting the backlinks many times in parallel leads to an OOM. With 37 million indexed items, I found that one requests consumes about 700MB of RAM in Solr, so with 5 GB of RAM, 10 parallel requests should lead to an OOM. Note that there are caches on the Solr side so you might need to try with different pages.

This also leads to slower than necessary refactoring operations.