Catalog Indexes

Catalogs are a flat file snapshot of a database state. They are loaded into memory as a BTreeMap and flushed to disk only after a flush method is explicitly called, thus allowing a form of batching, and implementation flexibility over managed behavior, which in some ways is preferable over an embedded database, for example.

Catalogs will facilitate chunking of large files into, say, 16MB chunks, and also, chunks help with parallelization of codec tasks over `num_cpus`.

Catalogs should have a struct that contains a BTreeMap, expose all its methods, and include `new` and `flush` utility methods on it.

Maybe in a later issue, a flush can be performed on drop (but only after checking explicitly flushed state), but this is a lot of work and introduces some magical behavior.