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/helpers.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 
+	"github.com/zrepl/zrepl/daemon/filters"
 	"github.com/zrepl/zrepl/platformtest"
 	"github.com/zrepl/zrepl/util/limitio"
 	"github.com/zrepl/zrepl/zfs"
@@ -174,3 +175,8 @@ func datasetToStringSortedTrimPrefix(prefix *zfs.DatasetPath, paths []*zfs.Datas
 	sort.Strings(pstrs)
 	return pstrs
 }
+
+func mustAddToSFilter(ctx *platformtest.Context, f *filters.DatasetMapFilter, fs string) {
+	err := f.Add(fs, "ok")
+	require.NoError(ctx, err)
+}