make initial replication policy configurable (most_recent, all, fail)

Config:

```
- type: push
  ...
  conflict_resolution:
    initial_replication: most_recent | all | fali
```

The ``initial_replication`` option determines which snapshots zrepl
replicates if the filesystem has not been replicated before.
If ``most_recent`` (the default), the initial replication will only
transfer the most recent snapshot, while ignoring previous snapshots.
If all snapshots should be replicated, specify ``all``.
Use ``fail`` to make replication of the filesystem fail in case
there is no corresponding fileystem on the receiver.

Code-Level Changes, apart from the obvious:
- Rework IncrementalPath()'s return signature.
  Now returns an error for initial replications as well.
- Rename & rework it's consumer, resolveConflict().

Co-authored-by: Graham Christensen <graham@grahamc.com>

Fixes https://github.com/zrepl/zrepl/issues/550
Fixes https://github.com/zrepl/zrepl/issues/187
Closes https://github.com/zrepl/zrepl/pull/592

replication/logic/replication_logic_policy.go

@@ -1,6 +1,9 @@
 package logic
 
 import (
+	"fmt"
+	"strings"
+
 	"github.com/go-playground/validator"
 	"github.com/pkg/errors"
 
@@ -8,16 +11,74 @@ import (
 	"github.com/zrepl/zrepl/replication/logic/pdu"
 )
 
+//go:generate enumer -type=InitialReplicationAutoResolution -trimprefix=InitialReplicationAutoResolution
+type InitialReplicationAutoResolution uint32
+
+const (
+	InitialReplicationAutoResolutionMostRecent InitialReplicationAutoResolution = 1 << iota
+	InitialReplicationAutoResolutionAll
+	InitialReplicationAutoResolutionFail
+)
+
+var initialReplicationAutoResolutionConfigMap = map[InitialReplicationAutoResolution]string{
+	InitialReplicationAutoResolutionMostRecent: "most_recent",
+	InitialReplicationAutoResolutionAll:        "all",
+	InitialReplicationAutoResolutionFail:       "fail",
+}
+
+func InitialReplicationAutoResolutionFromConfig(in string) (InitialReplicationAutoResolution, error) {
+	for v, s := range initialReplicationAutoResolutionConfigMap {
+		if s == in {
+			return v, nil
+		}
+	}
+	l := make([]string, 0, len(initialReplicationAutoResolutionConfigMap))
+	for _, v := range InitialReplicationAutoResolutionValues() {
+		l = append(l, initialReplicationAutoResolutionConfigMap[v])
+	}
+	return 0, fmt.Errorf("invalid value %q, must be one of %s", in, strings.Join(l, ", "))
+}
+
+type ConflictResolution struct {
+	InitialReplication InitialReplicationAutoResolution
+}
+
+func (c *ConflictResolution) Validate() error {
+	if !c.InitialReplication.IsAInitialReplicationAutoResolution() {
+		return errors.Errorf("must be one of %s", InitialReplicationAutoResolutionValues())
+	}
+	return nil
+}
+
+func ConflictResolutionFromConfig(in *config.ConflictResolution) (*ConflictResolution, error) {
+
+	initialReplication, err := InitialReplicationAutoResolutionFromConfig(in.InitialReplication)
+	if err != nil {
+		return nil, errors.Errorf("field `initial_replication` is invalid: %q is not one of %v", in.InitialReplication, InitialReplicationAutoResolutionValues())
+	}
+
+	return &ConflictResolution{
+		InitialReplication: initialReplication,
+	}, nil
+}
+
 type PlannerPolicy struct {
-	EncryptedSend             tri // all sends must be encrypted (send -w, and encryption!=off)
-	ReplicationConfig         *pdu.ReplicationConfig
-	SizeEstimationConcurrency int `validate:"gte=1"`
+	EncryptedSend             tri                    // all sends must be encrypted (send -w, and encryption!=off)
+	ConflictResolution        *ConflictResolution    `validate:"ne=nil"`
+	ReplicationConfig         *pdu.ReplicationConfig `validate:"ne=nil"`
+	SizeEstimationConcurrency int                    `validate:"gte=1"`
 }
 
 var validate = validator.New()
 
 func (p PlannerPolicy) Validate() error {
-	return validate.Struct(p)
+	if err := validate.Struct(p); err != nil {
+		return err
+	}
+	if err := p.ConflictResolution.Validate(); err != nil {
+		return err
+	}
+	return nil
 }
 
 func ReplicationConfigFromConfig(in *config.Replication) (*pdu.ReplicationConfig, error) {