Add yields to myrocks clone

Summary: After adding yields for chunks in the main loop, this is the only yield stack remaining on the clone path: https://fburl.com/scuba/mysql_thread_pool_stalls/jibfj8qd. To address them this change adds yields to the loops found in `donor::copy()`.

Differential Revision: D62220291

fbshipit-source-id: 63132134baf1749bb44df56b07e51390ac152143

storage/rocksdb/clone/donor.cc

@@ -834,6 +834,9 @@ int donor::copy(const THD *thd, uint task_id, Ha_clone_cbk &cbk) {
     auto err = handle_any_error(thd);
     if (err != 0) return err;
 
+    // This loop can take a long time so make sure to yield.
+    thd_check_yield(nullptr);
+
 #ifdef __APPLE__
     if (use_direct_io && fcntl(fd, F_NOCACHE, 1) == -1) {
       const auto errn = my_errno();
@@ -890,6 +893,9 @@ int donor::copy(const THD *thd, uint task_id, Ha_clone_cbk &cbk) {
         return err;
       }
 
+      // This loop can take a long time so make sure to yield.
+      thd_check_yield(nullptr);
+
       const auto chunk_size = metadata.chunk_size(buf_size);
       donor_chunk_metadata chunk{metadata.get_id(), chunk_size};
       myrocks::Rdb_string_writer buf;