* Removes all usage of block_on, and use a oneshot channel instead.
Calling `block_on` panics in certain context.
For instance, it panics when it is called in a the context of another
call to block.
Using it in tantivy is unnecessary. We replace it by a thin wrapper
around a oneshot channel that supports both async/sync.
* Removing needless uses of async in the API.
Co-authored-by: PSeitz <PSeitz@users.noreply.github.com>
