replication + endpoint: replication guarantees: guarantee_{resumability,incremental,nothing}

This commit

- adds a configuration in which no step holds, replication cursors, etc. are created
- removes the send.step_holds.disable_incremental setting
- creates a new config option `replication` for active-side jobs
- adds the replication.protection.{initial,incremental} settings, each
  of which can have values
    - `guarantee_resumability`
    - `guarantee_incremental`
    - `guarantee_nothing`
  (refer to docs/configuration/replication.rst for semantics)

The `replication` config from an active side is sent to both endpoint.Sender and endpoint.Receiver
for each replication step. Sender and Receiver then act accordingly.

For `guarantee_incremental`, we add the new `tentative-replication-cursor` abstraction.
The necessity for that abstraction is outlined in https://github.com/zrepl/zrepl/issues/340.

fixes https://github.com/zrepl/zrepl/issues/340

daemon/job/build_jobs_sendrecvoptions.go

@@ -0,0 +1,55 @@
+package job
+
+import (
+	"github.com/pkg/errors"
+	"github.com/zrepl/zrepl/config"
+	"github.com/zrepl/zrepl/daemon/filters"
+	"github.com/zrepl/zrepl/endpoint"
+	"github.com/zrepl/zrepl/zfs"
+)
+
+type SendingJobConfig interface {
+	GetFilesystems() config.FilesystemsFilter
+	GetSendOptions() *config.SendOptions // must not be nil
+}
+
+func buildSenderConfig(in SendingJobConfig, jobID endpoint.JobID) (*endpoint.SenderConfig, error) {
+
+	fsf, err := filters.DatasetMapFilterFromConfig(in.GetFilesystems())
+	if err != nil {
+		return nil, errors.Wrap(err, "cannot build filesystem filter")
+	}
+
+	return &endpoint.SenderConfig{
+		FSF:     fsf,
+		Encrypt: &zfs.NilBool{B: in.GetSendOptions().Encrypted},
+		JobID:   jobID,
+	}, nil
+}
+
+type ReceivingJobConfig interface {
+	GetRootFS() string
+	GetAppendClientIdentity() bool
+	GetRecvOptions() *config.RecvOptions
+}
+
+func buildReceiverConfig(in ReceivingJobConfig, jobID endpoint.JobID) (rc endpoint.ReceiverConfig, err error) {
+	rootFs, err := zfs.NewDatasetPath(in.GetRootFS())
+	if err != nil {
+		return rc, errors.New("root_fs is not a valid zfs filesystem path")
+	}
+	if rootFs.Length() <= 0 {
+		return rc, errors.New("root_fs must not be empty") // duplicates error check of receiver
+	}
+
+	rc = endpoint.ReceiverConfig{
+		JobID:                      jobID,
+		RootWithoutClientComponent: rootFs,
+		AppendClientIdentity:       in.GetAppendClientIdentity(),
+	}
+	if err := rc.Validate(); err != nil {
+		return rc, errors.Wrap(err, "cannot build receiver config")
+	}
+
+	return rc, nil
+}