Exact term queries behave differently against the DB and Solr

Steps to reproduce

1. Index a document with cm:name set to Taxi Driver.
2. Send an AFTS query to Solr for =cm:name:Driver and check that the document is returned.
3. Send an AFTS query via the v1 REST API for =cm:name:Driver

Expected result
4. The document is returned.

Actual result
4. The document is not returned.

Investigation

This is caused by a difference in behaviour of queries handled by the DB and queries handled by SS 2.0.

If we modify the query slightly, e.g. to =cm:name:Driver AND cm:name:* then the query is redirected to Solr and the document is returned.

Notes

This issue was raised by Axel here: https://hub.alfresco.com/t5/alfresco-content-services-blog/exact-term-queries-in-search-services-2-0/bc-p/302201/highlight/true#M2474

This could either be seen as a regression (queries behave differently depending on where they are handled) or as an existing issue (DB queries have never handled exact term searches as documented).