Caching on execute_loaders

[rochacbruno]

Currently, all loaders are executed again every time we

• switch namespaces using settings.using_namespace or settings.namespace
• when a variable is marked as always_fresh
• when using `settings.get('var', fresh=True)

For the fresh=True and always_fresh it makes sense to execute loaders again as it is demanded to get the fresh variable (lets say from redis or another database).

The problem is that it may cause overhead (I guess not too much) but in any case we can improve by.

• writing more namespace tests (with more switching and fresh examples)
• implement a cache on execute_loaders so files and databases are not accessed twice when we are only switching namespaces and not wanting to load all again.

Part of this is already implemented in loaded_by_loaders dictionary, we just need to use this as a cache point.

[rochacbruno]

Closing as this is a no-issue