fix data race related to VersionExists (#36)

* fix data race related to VersionExists

* use regular lock instead of RW in mutable_tree.go

mutable_tree.go

@@ -37,7 +37,7 @@ type MutableTree struct {
 	unsavedFastNodeRemovals  map[string]interface{} // FastNodes that have not yet been removed from disk
 	ndb                      *nodeDB
 
-	mtx sync.RWMutex // versions Read/write lock.
+	mtx sync.Mutex
 }
 
 // NewMutableTree returns a new tree with the specified cache size and datastore.
@@ -88,8 +88,8 @@ func (tree *MutableTree) VersionExists(version int64) bool {
 
 // AvailableVersions returns all available versions in ascending order
 func (tree *MutableTree) AvailableVersions() []int {
-	tree.mtx.RLock()
-	defer tree.mtx.RUnlock()
+	tree.mtx.Lock()
+	defer tree.mtx.Unlock()
 
 	res := make([]int, 0, len(tree.versions))
 	for i, v := range tree.versions {