Reverse entity references don't work with fields referencing multiple entities

Problem/Motivation

The reverse entity reference processor does not seem to work with fields referencing multiple entities.

Steps to reproduce

Create an entity A with field foo that contains an unlimited number of references to entities of type B. The search API allows the user to set up indexing reverse entity references but they are not indexed.

Proposed resolution

A closer inspection of the code reveals that, to my understanding at least, support for this does not exist in the plugin code. The referenced condition statement compares the value of the reference field with the provided entity ID which is bound to not work in cases of fields referencing multiple entities. I am not closely familiar with how Drupal stores entity references but it seems that a more complex lookup is required here, one that takes into account the possibility of fields referencing multiple entities (i.e. a table join may be necessary).