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

daemon/job/active.go

@@ -162,8 +162,14 @@ func modePushFromConfig(g *config.Global, in *config.PushJob, jobID endpoint.Job
 		return nil, errors.Wrap(err, "field `replication`")
 	}
 
+	conflictResolution, err := logic.ConflictResolutionFromConfig(in.ConflictResolution)
+	if err != nil {
+		return nil, errors.Wrap(err, "field `conflict_resolution`")
+	}
+
 	m.plannerPolicy = &logic.PlannerPolicy{
 		EncryptedSend:             logic.TriFromBool(in.Send.Encrypted),
+		ConflictResolution:        conflictResolution,
 		ReplicationConfig:         replicationConfig,
 		SizeEstimationConcurrency: in.Replication.Concurrency.SizeEstimates,
 	}
@@ -261,8 +267,14 @@ func modePullFromConfig(g *config.Global, in *config.PullJob, jobID endpoint.Job
 		return nil, errors.Wrap(err, "field `replication`")
 	}
 
+	conflictResolution, err := logic.ConflictResolutionFromConfig(in.ConflictResolution)
+	if err != nil {
+		return nil, errors.Wrap(err, "field `conflict_resolution`")
+	}
+
 	m.plannerPolicy = &logic.PlannerPolicy{
 		EncryptedSend:             logic.DontCare,
+		ConflictResolution:        conflictResolution,
 		ReplicationConfig:         replicationConfig,
 		SizeEstimationConcurrency: in.Replication.Concurrency.SizeEstimates,
 	}