Fix failed test

Summary: Fix failed persistent cache test introduced by recent PR to support cross-platform build.

Test Plan: Run persistent_cache_test (on multiple platforms)

Reviewers: sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D61467

utilities/persistent_cache/block_cache_tier.h

@@ -68,11 +68,8 @@ class BlockCacheTier : public PersistentCacheTier {
 
   void TEST_Flush() override {
     while (insert_ops_.Size()) {
-#ifdef OS_WIN
-      Sleep(1000);
-#else
-      /* sleep override */ sleep(1);
-#endif
+      /* sleep override */
+      Env::Default()->SleepForMicroseconds(1000000);
     }
   }
 

utilities/persistent_cache/persistent_cache_tier.h

@@ -200,7 +200,8 @@ struct PersistentCacheConfig {
   // file size in order to avoid dead lock.
   size_t write_buffer_count() const {
     assert(write_buffer_size);
-    return (writer_qdepth + writer_qdepth/5) * cache_file_size / write_buffer_size;
+    return static_cast<size_t>((writer_qdepth + 1.2) * cache_file_size /
+                               write_buffer_size);
   }
 
   // writer-dispatch-size