Use 'toArray()' in addition to array access after 'find()' in rslib.js

If we use coll.find().toArray()[0] instead of just coll.find()[0] then we will be able to catch the CappedPositionLost error as expected in getFirstOplogEntry().

(This doesn't solve the direct cause of the bug in the linked ticket, but it will avoid the bug in the future.)