tape: media_catalog: add snapshot list cache for catalog

For some parts of the ui, we only need the snapshot list from the catalog,
and reading the whole catalog (can be multiple hundred MiB) is not
really necessary.

Instead, we write the list of snapshots into a seperate .index file. This file
is generated on demand and is much smaller and thus faster to read.

src/tape/mod.rs

@@ -42,6 +42,9 @@ pub use media_pool::*;
 mod media_catalog;
 pub use media_catalog::*;
 
+mod media_catalog_cache;
+pub use media_catalog_cache::*;
+
 mod pool_writer;
 pub use pool_writer::*;