storage: Make locking an implementation detail for #69

- file: Store handles its own locking #77
- file: Move mbox into its own file
- file & test: remove LockFor()

pkg/test/storage.go

@@ -2,7 +2,6 @@ package test
 
 import (
 	"errors"
-	"sync"
 
 	"github.com/jhillyerd/inbucket/pkg/storage"
 )
@@ -82,12 +81,6 @@ func (s *StoreStub) VisitMailboxes(f func([]storage.StoreMessage) (cont bool)) e
 	return nil
 }
 
-// LockFor mock function returns a new RWMutex, never errors.
-// TODO(#69) remove
-func (s *StoreStub) LockFor(name string) (*sync.RWMutex, error) {
-	return &sync.RWMutex{}, nil
-}
-
 // MessageDeleted returns true if the specified message was deleted
 func (s *StoreStub) MessageDeleted(m storage.StoreMessage) bool {
 	_, ok := s.deleted[m]