Mixed encryption states with `mc mirror --encrypt-key`

[djwfyi]

Summary

Referring to @ravindk89 comment on PR442, confirm how we handle en/decrypt when objects on the source drive are mixed encrypted states (some are encrypted, others are not).

Does everything get encrypted? Does everything get decrypted? How do they wind up on the target?

For simplicity, assuming encryption is handled with the same key.

Goals

List the in-scope goals

• [ ] Understand mixed-state encryption
• [ ] Update the mc mirror doc as needed