R1:4f647fb6be2b

When updating a Ferret search index document, reuse existing rows where possible

Summary:
Ref T13587. Currently, when a document is reindexed by Ferret, the old document is completely discarded and a new version is inserted to replace it.

This approach is simple to implement, but can lead to exhaustion of the ngram AUTO_INCREMENT id column in reasonable circumstances.

Conceptually, this approach "should" be fine and this exhaustion is an awkard implementation detail. However, since it's easy to be less…