fix encrypt-on-receive with placeholders

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

platformtest/tests/generated_cases.go

@@ -24,6 +24,9 @@ var Cases = []Case{BatchDestroy,
 	ReplicationIsResumableFullSend__both_GuaranteeResumability,
 	ReplicationIsResumableFullSend__initial_GuaranteeIncrementalReplication_incremental_GuaranteeIncrementalReplication,
 	ReplicationIsResumableFullSend__initial_GuaranteeResumability_incremental_GuaranteeIncrementalReplication,
+	ReplicationPlaceholderEncryption__EncryptOnReceiverUseCase__WorksIfConfiguredWithInherit,
+	ReplicationPlaceholderEncryption__UnspecifiedIsOkForClientIdentityPlaceholder,
+	ReplicationPlaceholderEncryption__UnspecifiedLeadsToFailureAtRuntimeWhenCreatingPlaceholders,
 	ReplicationPropertyReplicationWorks,
 	ReplicationReceiverErrorWhileStillSending,
 	ReplicationStepCompletedLostBehavior__GuaranteeIncrementalReplication,