This change makes it so that mongod will set the first "repairable" feature bit (named kPathLevelMultikeyTracking) to indicate that at least one index in the data files supports path-level multikey tracking. If no feature has already been marked as in-use, then creating a new index on 3.3.7+ will also cause a feature document to be inserted into the KVCatalog.
When attempting to downgrade to a version of MongoDB earlier than 3.2.7 (i.e. a mongod without the changes from
SERVER-23117), the mongod will fail to start with the following message:
The feature bit in the KVCatalog should only be set when a new index is being created on 3.3+. The changes in this ticket will make it so that it isn't possible to downgrade to a version older of MongoDB than the fixVersion of