Rewrite SyncClient to use AsyncClient internally

[theduke]

To prevent code duplication, the SyncClient should be rewritten to just use the async client internally.

[joseluis]

This sounds good but do you mind if I do some PR fixing & adding some more functionality to the sync part first?

I'm close to releasing a crate I'm working on which has this project as a base dependency and I'd like to work on enabling the search filters, impl Default om some types (Sort, ListOptions), adding some more missing fields under Crate...

[theduke]

Sure no problem, as long as you keep functionality in sync between sync and async versions.