3.6 MongoRocks PeriodicKillSecondaries:ValidateCollections test hook fails

Here is the description of what could happen before the fix for this issue:
In some specific conditions data records could disappear at arbitrary moment of time from the collections or indexes created after server restart. This could happen as the result of the following sequence of events:
1. User deletes one or more indexes or collections. These should be the ones using maximum existing prefixes values.
2. User shuts down the server before mongo-rocks compaction thread executes compactions of deleted ranges.
3. User restarts the server and creates some new collections. Due to the bug those new collections and their indexes may get the same prefix values which were deleted and not yet compacted. User inserts some data into the new collections.
4. After the server restart mongo-rocks compaction thread continues executing compactions of the deleted ranges and this process may eventually delete data from the collections sharing prefixes with deleted ranges.

There is a bug in mongo-rocks engine initialization code which causes wrong initialization of _maxPrefix value. This may lead to reuse of dropped prefix and to accidental remove of data from the collection using reused prefix. The same issue exists in v3.2 and v3.4.

I made a comparison for this suite between 3.4 and 3.6 and I don't see the issue with 3.4. Here is the summary for 50 test runs for both versions. If needed I can give access to test logs since they're to big to upload to Jira.

3.4:

3.6:

Can you please work on this?