feat: add upstream and verbose flags to command line interface

feat: add upstream support allowing to chain cache servers if needed
fix: tweaked garbage collection to be better

vfs/vfserror/vfserror.go

@@ -3,6 +3,9 @@ package vfserror
 import "errors"
 
 var (
+	// ErrInvalidKey is returned when a key is invalid.
+	ErrInvalidKey = errors.New("vfs: invalid key")
+
 	// ErrUnreachable is returned when a code path is unreachable.
 	ErrUnreachable = errors.New("unreachable")