storage/Interface: add virtual method OpenFile()

This should replace most InputStream::Open() calls because it is a
chance to reuse existing resources (e.g. the NFS connection).  No such
optimization is implemented, currently (and the method is not yet used
by anybody).

test/meson.build

@@ -214,6 +214,7 @@ if enable_database
   executable(
     'run_storage',
     'run_storage.cxx',
+    '../src/TagSave.cxx',
     include_directories: inc,
     dependencies: [
       event_dep,