CDRIVER-3893: Conditionally support $merge and $out execution on secondaries (#904)

* Allow readPreference:secondary with $merge and $out. [Finish CDRIVER-3893, CDRIVER-4224, and CDRIVER-4195]
* Pass around the effectively chosen read mode used in server selection back to the command builder
* Update doc func name
* Fix duplicate doc comments
* Consolidate duplicate state in suitable_servers()
* Less magic numbers
* Clarify wire version checks
* Force primary read preference if talking to a single server older than 5.0