ALTER DATABASE SET ALLOW_SNAPSHOT_ISOLATION ON takes forever
I ran into this exact situation and can confirm there was no blocking in my case as the setup basically altered the snapshot isolation immediately after creating the database.

In my case the wait type was ENABLE_VERSIONING.

When investigating I found this feedback item Enabling snapshot isolation in 2016 SP1 CU6 and CU7 hangs which is still under investigation.

One of the comments seem to suggest a SERVER AUDIT could be the case.

If you don't have a server audit running I'm afraid your option for now will be to roll back to an older CU and watch that feedback item for updates.