Make cache-db.json into a binary file #4010

Summary

Replace cache-db.json with cache-db.cbor, using the same mechanism for serialization as we already do for externs files.

Motivation

The correctness of the incremental build system depends on an assumption that only the compiler is doing anything with cache-db files, so avoiding JSON is desirable because it also decreases the likelihood that other projects might start depending on them. It should help speed up serialization and deserialization a bit too.