Removes all usage of block_on, and use a oneshot channel instead. (#1315)

* 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>

src/query/term_query/term_scorer.rs

@@ -125,7 +125,6 @@ impl Scorer for TermScorer {
 
 #[cfg(test)]
 mod tests {
-    use futures::executor::block_on;
     use proptest::prelude::*;
 
     use crate::merge_policy::NoMergePolicy;
@@ -321,9 +320,7 @@ mod tests {
                 .collect();
             test_block_wand_aux(&term_query, &searcher)?;
         }
-        {
-            let _ = block_on(writer.merge(&segment_ids[..]));
-        }
+        writer.merge(&segment_ids[..]).wait().unwrap();
         {
             reader.reload()?;
             let searcher = reader.searcher();