[AL-1619][AL-1586] Removes cachable, makes Info more dict like

[AbhinavTuli]

🚀 🚀 Pull Request

Checklist:

• My code follows the style guidelines of this project and the Contributing document
• I have commented my code, particularly in hard-to-understand areas
• I have kept the coverage-rate up
• I have performed a self-review of my own code and resolved any problems
• I have checked to ensure there aren't any other open Pull Requests for the same change
• I have described and made corresponding changes to the relevant documentation
• New and existing unit tests pass locally with my changes

Changes

• Removes Cachable, should increase stability, reduce memory leaks and speed up certain writes.
• Info object is now much more dict like and supports things like:-

ds.info.keys()
ds.info.values()
ds.info.items()
"xyz" in ds.info
ds.info.setdefault(0, "hello")
ds.info.popitem()
ds.info.pop(0)
del ds.info["abc"]
for it in ds.info:
    pass

[jraman]

Since this change doesn't look to be backward compatible, hub version needs to be appropriately bumped.

[AbhinavTuli]

This is a slight change in API for info, delete is barely used atm, we should be fine.

[jraman]

Separate tests for some of these methods (like popitem) might be better -- LIFO popitem not obvious here :)

[farizrahman4u]

In python < 3.7, popitem() pops and returns an arbitrary entry while in 3.7+ it returns LIFO. Have you taken that into account wherever popitem is called? (Not just info, but lru_sizes.popitem etc as well)

[AbhinavTuli]

For info, it shouldn't matter, it will behave like the corresponding version's dictionary.
For LRUCache, I don't think it should be an issue even in 3.6 as the Cpython implementation retains order.

From StackOverflow (https://stackoverflow.com/questions/39980323/are-dictionaries-ordered-in-python-3-6):-
"They are insertion ordered. As of Python 3.6, for the CPython implementation of Python, dictionaries remember the order of items inserted. This is considered an implementation detail in Python 3.6; you need to use OrderedDict if you want insertion ordering that's guaranteed across other implementations of Python (and other ordered behavior).

As of Python 3.7, this is no longer an implementation detail and instead becomes a language feature."

changes made