fixes https://github.com/zrepl/zrepl/issues/504
Problem:
plain send + recv with root_fs encrypted + placeholders causes plain recvs
whereas user would expect encrypt-on-recv
Reason:
We create placeholder filesytems with -o encryption=off.
Thus, children received below those placeholders won't inherit
encryption of root_fs.
Fix:
We'll have three values for `recv.placeholders.encryption: unspecified (default) | off | inherit`.
When we create a placeholder, we will fail the operation if `recv.placeholders.encryption = unspecified`.
The exception is if the placeholder filesystem is to encode the client identity ($root_fs/$client_identity) in a pull job.
Those are created in `inherit` mode if the config field is `unspecified` so that users who don't need
placeholders are not bothered by these details.
Future Work:
Automatically warn existing users of encrypt-on-recv about the problem
if they are affected.
The problem that I hit during implementation of this is that the
`encryption` prop's `source` doesn't quite behave like other props:
`source` is `default` for `encryption=off` and `-` when `encryption=on`.
Hence, we can't use `source` to distinguish the following 2x2 cases:
(1) placeholder created with explicit -o encryption=off
(2) placeholder created without specifying -o encryption
with
(A) an encrypted parent at creation time
(B) an unencrypted parent at creation time
