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/state_enumer.go

@@ -0,0 +1,50 @@
+// Code generated by "enumer -type=State"; DO NOT EDIT.
+
+package fsrep
+
+import (
+	"fmt"
+)
+
+const _StateName = "ReadyCompleted"
+
+var _StateIndex = [...]uint8{0, 5, 14}
+
+func (i State) String() string {
+	i -= 1
+	if i >= State(len(_StateIndex)-1) {
+		return fmt.Sprintf("State(%d)", i+1)
+	}
+	return _StateName[_StateIndex[i]:_StateIndex[i+1]]
+}
+
+var _StateValues = []State{1, 2}
+
+var _StateNameToValueMap = map[string]State{
+	_StateName[0:5]:  1,
+	_StateName[5:14]: 2,
+}
+
+// StateString retrieves an enum value from the enum constants string name.
+// Throws an error if the param is not part of the enum.
+func StateString(s string) (State, error) {
+	if val, ok := _StateNameToValueMap[s]; ok {
+		return val, nil
+	}
+	return 0, fmt.Errorf("%s does not belong to State values", s)
+}
+
+// StateValues returns all values of the enum
+func StateValues() []State {
+	return _StateValues
+}
+
+// IsAState returns "true" if the value is listed in the enum definition. "false" otherwise
+func (i State) IsAState() bool {
+	for _, v := range _StateValues {
+		if i == v {
+			return true
+		}
+	}
+	return false
+}