Allow disabling of conversion of cache data

@jameskleeh is already working on this.

Since Feb 18, 2020.

### Task List

- [x] Steps to reproduce provided

### Steps to Reproduce

1. Write a method which returns a huge map and annotate it with `@Cacheable`
1. Call that method
1. Call it again

### Expected Behaviour

2nd method call is at least as fast as 1st.

### Actual Behaviour

2nd method call is a lot slower.

### Environment Information

- **Operating System**: Fedora 31
- **Micronaut Version:** 1.3.0
- **JDK Version:** AdoptJDK 11.0.6

Hi,

the DefaultSyncCache uses [the conversion service](https://github.com/micronaut-projects/micronaut-core/blob/1.3.x/runtime/src/main/java/io/micronaut/cache/DefaultSyncCache.java#L56) to [convert the stuff](https://github.com/micronaut-projects/micronaut-core/blob/1.3.x/runtime/src/main/java/io/micronaut/cache/DefaultSyncCache.java#L124) coming from the actual cache implementation. The `DefaultConversionService` contains some code which iterates over all the keys from map (because it implements Iterable) and tries to convert them. I'm not sure why it's doing this, but this caused a huge performance hit in our application. We're using the default in-memory caffeine cache.

We worked around by wrapping the huge map in an object which doesn't implement Iterable and now it's fast again.