[PHP-638] Return constant string types in getReadPreference()

See: https://jira.mongodb.org/browse/PHP-638

Replace numeric types with the constant string values and eliminate the irrelevant "type_string" field.

The change to `mongo_read_preference_type_to_name()` should also fix compatibility with mongos, as the "preferred" strings were not what the server expected in the cursor `$readPreference` mode.

Note: this will pose a BC break for any code that was depending on numeric values in the "type" element, or reading the "type_string" element.