OAK-1641: Mongo: Un-/CheckedExecutionException on replica-primary crash

- Default to ReadPreference.primary() unless read from secondary is safe. 
- Updated tests accordingly
- Introduce a simply retry for uncached find call

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1622239 13f79535-47bb-0310-9956-ffa450edef68