refactoring codebase

[amirreza-sf80]

this is a sub-pr from #24

1. refactor the code base to reduce repeated code
2. add utils for raw usage
3. sharded client now has get_client, but get_server is removed

NOTES:

refactoring invloves these points:

1.moving all client operations into their own class
2.removing shared methods from other clients, and handling all of them in one method (where possible)
3. removing hardcoded (a) prifix from async code and handling that dynamically
4. turning some async methods into sync so both clients can share them
5. reuse normal sync tests for cluster (as much as possible)
6. document the changes

[amirreza-sf80]

note for future:
i've tested removing methods from the cache backend and replace them with a __getattr__
but i won't add it to this PR, i need more time to think about it