Add IsDirectory() to Env and FS (#6711)

Summary:
IsDirectory() is a common API to check whether a path is a regular file or
directory.
POSIX: call stat() and use S_ISDIR(st_mode)
Windows: PathIsDirectoryA() and PathIsDirectoryW()
HDFS: FileSystem.IsDirectory()
Java: File.IsDirectory()
...
Pull Request resolved: facebook/rocksdb#6711

Test Plan: make check

Reviewed By: anand1976

Differential Revision: D21053520

Pulled By: riversand963

fbshipit-source-id: 680aadfd8ce982b63689190cf31b3145d5a89e27
Signed-off-by: Changlong Chen <levisonchen@live.cn>

include/rocksdb/file_system.h

@@ -522,6 +522,10 @@ class FileSystem {
     return IOStatus::NotSupported();
   }
 
+  virtual IOStatus IsDirectory(const std::string& /*path*/,
+                               const IOOptions& options, bool* is_dir,
+                               IODebugContext* /*dgb*/) = 0;
+
   // If you're adding methods here, remember to add them to EnvWrapper too.
 
  private:
@@ -1193,6 +1197,10 @@ class FileSystemWrapper : public FileSystem {
                         uint64_t* diskfree, IODebugContext* dbg) override {
     return target_->GetFreeSpace(path, options, diskfree, dbg);
   }
+  IOStatus IsDirectory(const std::string& path, const IOOptions& options,
+                       bool* is_dir, IODebugContext* dbg) override {
+    return target_->IsDirectory(path, options, is_dir, dbg);
+  }
 
  private:
   std::shared_ptr<FileSystem> target_;