Extract template registry to its own class

[yaauie]

This PR reduces the complexity of creating, persisting, loading, and retrieving BinData::Struct instances from the local- and file-based caches by extracting the relevant code and the necessary threadsafety mutexes into a new thread-safe class called TemplateRegistry.

This new class has two main entry points:

• TemplateRegistry.register(key, fields): in a thread-safe manner and respecting the codec's @cache_ttl, register the given list of fields as a new BinData::Struct, persisting to disk if configured to do so.
• TemplateRegistry.fetch(key): in a thread-safe manner, retrieve the BinData::Struct that has been registered with the given key.

It uses the existing tests to prove itself.

There is a minor change in behaviour to this implementation that surfaces in non-happy-path scenarios: when encountering a corrupt file, an unwritable file, or any other exception reading or writing the file, this implementation logs an error and continues instead of crashing the codec and potentially its input.

[jorritfolmer]

Awesome! Thanks for the rework!
Performance is up +10% with 2 workers.