fetch: fix the nested async runtime caused by block_on()

Summary:
Currently, we have two commton `block_on`s:
* 1st block_on is in python bindlings for converting Rust async to sync
* 2nd block_on is in FetchState.fetch_cas()

This diff adds `block_in_place` for the `block_on()` usage inside FetchState.fetch_cas(), so it can be
called in an async runtime.

Reviewed By: quark-zju

Differential Revision: D65861552

fbshipit-source-id: bce677fe86e83db705f31a356829c426ebd743bd

eden/scm/lib/revisionstore/src/scmstore/tree/fetch.rs

@@ -229,8 +229,9 @@ impl FetchState {
         let start_time = Instant::now();
         let mut total_stats = CasFetchedStats::default();
 
-        block_on(async {
-            cas_client.fetch(&digests, CasDigestType::Tree).await.for_each(|results| match results {
+        async_runtime::block_in_place(|| {
+            block_on(async {
+                cas_client.fetch(&digests, CasDigestType::Tree).await.for_each(|results| match results {
                 Ok((stats, results)) => {
                     reqs += 1;
                     total_stats.add(&stats);
@@ -309,6 +310,7 @@ impl FetchState {
                     future::ready(())
                 }
             }).await;
+            })
         });
 
         span.record("hits", keys_found_count);