Re-implement Serialize and Deserialize for Catalog as is done for its nested types

[hiltontj]

Problem

The Catalog type does not implement the Deserialize trait, which means that if we try to nest it in another struct that needs to implement Deserialize, then we run into headaches.

Proposed solution

There exist several _Snapshot types in the serialize.rs module that provide the minimum needed info for de/serialization of their related type in the catalog, e.g., DatabaseSchema/DatabaseSnapshot, TableDefinition/TableSnapshot, etc.

I propose that we re-use this pattern at the top level, and define a CatalogSnapshot in the same serialize.rs module, along with a impl Deserialize/impl Serialize for the Catalog type that uses it.

Alternatives

• Implement Deserialize for Catalog as is. This should be as simple as

// current code:
#[derive(Debug)]
pub struct Catalog {
    inner: RwLock<InnerCatalog>,
}

// change to:
#[derive(Debug, Deserialize, Serialize)]
pub struct Catalog {
    #[serde(flatten)] // <- this is important
    inner: RwLock<InnerCatalog>,
}

Additional context

The reason for the use of these _Snapshot types is to avoid serializing/deserializing the bi-directional maps of ID <-> name at the various levels of the catalog. Since the info in those maps is already intrinsic to the map of ID -> object (which includes the name), there is no need to also serialize/deserialize them. The top level is still serializing/deserializing the bi-directional map for DbId <-> name, so following through with this would avoid that.