Problem

https://github.com/mongodb/mongo/blob/master/src/mongo/db/commands/collection_to_capped.cpp#L217

            // calls renamecollection which does a global lock, so we must too:
            //
            Lock::GlobalWrite globalWriteLock(txn->lockState());
            Client::Context ctx(txn, dbname);

Per SERVER-15723, the lock taken for renameCollection may change, therefore this command should

• match any change made to renameCollection
• this probably only needs to be DB_X lock because convertToCapped does not take a namespace parameter