replication: rewrite error handling + simplify state machines

* Remove explicity state machine code for all but replication.Replication
* Introduce explicit error types that satisfy interfaces which provide
  sufficient information for replication.Replication to make intelligent
  retry + queuing decisions

  * Temporary()
  * LocalToFS()

* Remove the queue and replace it with a simple array that we sort each
  time (yay no generics :( )

replication/fsrep/stepstate_enumer.go

@@ -0,0 +1,61 @@
+// Code generated by "enumer -type=StepState"; DO NOT EDIT.
+
+package fsrep
+
+import (
+	"fmt"
+)
+
+const (
+	_StepStateName_0 = "StepReplicationReadyStepMarkReplicatedReady"
+	_StepStateName_1 = "StepCompleted"
+)
+
+var (
+	_StepStateIndex_0 = [...]uint8{0, 20, 43}
+	_StepStateIndex_1 = [...]uint8{0, 13}
+)
+
+func (i StepState) String() string {
+	switch {
+	case 1 <= i && i <= 2:
+		i -= 1
+		return _StepStateName_0[_StepStateIndex_0[i]:_StepStateIndex_0[i+1]]
+	case i == 4:
+		return _StepStateName_1
+	default:
+		return fmt.Sprintf("StepState(%d)", i)
+	}
+}
+
+var _StepStateValues = []StepState{1, 2, 4}
+
+var _StepStateNameToValueMap = map[string]StepState{
+	_StepStateName_0[0:20]:  1,
+	_StepStateName_0[20:43]: 2,
+	_StepStateName_1[0:13]:  4,
+}
+
+// StepStateString retrieves an enum value from the enum constants string name.
+// Throws an error if the param is not part of the enum.
+func StepStateString(s string) (StepState, error) {
+	if val, ok := _StepStateNameToValueMap[s]; ok {
+		return val, nil
+	}
+	return 0, fmt.Errorf("%s does not belong to StepState values", s)
+}
+
+// StepStateValues returns all values of the enum
+func StepStateValues() []StepState {
+	return _StepStateValues
+}
+
+// IsAStepState returns "true" if the value is listed in the enum definition. "false" otherwise
+func (i StepState) IsAStepState() bool {
+	for _, v := range _StepStateValues {
+		if i == v {
+			return true
+		}
+	}
+	return false
+}