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/initialreplicationautoresolution_enumer.go

@@ -0,0 +1,62 @@
+// Code generated by "enumer -type=InitialReplicationAutoResolution -trimprefix=InitialReplicationAutoResolution"; DO NOT EDIT.
+
+//
+package logic
+
+import (
+	"fmt"
+)
+
+const (
+	_InitialReplicationAutoResolutionName_0 = "MostRecentAll"
+	_InitialReplicationAutoResolutionName_1 = "Fail"
+)
+
+var (
+	_InitialReplicationAutoResolutionIndex_0 = [...]uint8{0, 10, 13}
+	_InitialReplicationAutoResolutionIndex_1 = [...]uint8{0, 4}
+)
+
+func (i InitialReplicationAutoResolution) String() string {
+	switch {
+	case 1 <= i && i <= 2:
+		i -= 1
+		return _InitialReplicationAutoResolutionName_0[_InitialReplicationAutoResolutionIndex_0[i]:_InitialReplicationAutoResolutionIndex_0[i+1]]
+	case i == 4:
+		return _InitialReplicationAutoResolutionName_1
+	default:
+		return fmt.Sprintf("InitialReplicationAutoResolution(%d)", i)
+	}
+}
+
+var _InitialReplicationAutoResolutionValues = []InitialReplicationAutoResolution{1, 2, 4}
+
+var _InitialReplicationAutoResolutionNameToValueMap = map[string]InitialReplicationAutoResolution{
+	_InitialReplicationAutoResolutionName_0[0:10]:  1,
+	_InitialReplicationAutoResolutionName_0[10:13]: 2,
+	_InitialReplicationAutoResolutionName_1[0:4]:   4,
+}
+
+// InitialReplicationAutoResolutionString retrieves an enum value from the enum constants string name.
+// Throws an error if the param is not part of the enum.
+func InitialReplicationAutoResolutionString(s string) (InitialReplicationAutoResolution, error) {
+	if val, ok := _InitialReplicationAutoResolutionNameToValueMap[s]; ok {
+		return val, nil
+	}
+	return 0, fmt.Errorf("%s does not belong to InitialReplicationAutoResolution values", s)
+}
+
+// InitialReplicationAutoResolutionValues returns all values of the enum
+func InitialReplicationAutoResolutionValues() []InitialReplicationAutoResolution {
+	return _InitialReplicationAutoResolutionValues
+}
+
+// IsAInitialReplicationAutoResolution returns "true" if the value is listed in the enum definition. "false" otherwise
+func (i InitialReplicationAutoResolution) IsAInitialReplicationAutoResolution() bool {
+	for _, v := range _InitialReplicationAutoResolutionValues {
+		if i == v {
+			return true
+		}
+	}
+	return false
+}