• Check mongo version for validation.
• Small refactors to use bson struct tags for deserialization.
• Extract shared logic to shared/mongo/validation.go
• Default read preference mode to SecondaryPreferred, which read from primary only if secondary does not exists -- we should still support read from primary via uri if user prefers, will work on this as a subsequent PR as we separate out username/password from rest of uri.

Test: locally tested deserialization.