[RocksDB] cleanup EnvOptions

Summary:
This diff simplifies EnvOptions by treating it as POD, similar to Options.
- virtual functions are removed and member fields are accessed directly.
- StorageOptions is removed.
- Options.allow_readahead and Options.allow_readahead_compactions are deprecated.
- Unused global variables are removed: useOsBuffer, useFsReadAhead, useMmapRead, useMmapWrite

Test Plan: make check; db_stress

Reviewers: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11175

include/leveldb/env.h

@@ -23,16 +23,40 @@
 namespace leveldb {
 
 class FileLock;
-class EnvOptions;
 class Logger;
 class RandomAccessFile;
 class SequentialFile;
 class Slice;
 class WritableFile;
+class Options;
 
 using std::unique_ptr;
 using std::shared_ptr;
 
+
+// Options while opening a file to read/write
+struct EnvOptions {
+
+  // construct with default Options
+  EnvOptions();
+
+  // construct from Options
+  EnvOptions(const Options& options);
+
+  // If true, then allow caching of data in environment buffers
+  bool use_os_buffer;
+
+   // If true, then use mmap to read data
+  bool use_mmap_reads;
+
+   // If true, then use mmap to write data
+  bool use_mmap_writes;
+
+  // If true, set the FD_CLOEXEC on open fd.
+  bool set_fd_cloexec;
+
+};
+
 class Env {
  public:
   Env() { }
@@ -375,27 +399,6 @@ class FileLock {
   void operator=(const FileLock&);
 };
 
-// Options while opening a file to read/write
-class EnvOptions {
-  public:
-   virtual ~EnvOptions() {}
-
-   // If true, then allow caching of data in environment buffers
-   virtual bool UseOsBuffer() const = 0;
-
-   // If true, then allow the environment to readahead data
-   virtual bool UseReadahead() const = 0;
-
-   // If true, then use mmap to read data
-   virtual bool UseMmapReads() const = 0;
-
-   // If true, then use mmap to write data
-   virtual bool UseMmapWrites() const = 0;
-
-   // If true, set the FD_CLOEXEC on open fd.
-   virtual bool IsFDCloseOnExec() const = 0;
-};
-
 // Log the specified data to *info_log if info_log is non-nullptr.
 extern void Log(const shared_ptr<Logger>& info_log, const char* format, ...)
 #   if defined(__GNUC__) || defined(__clang__)

include/leveldb/options.h

@@ -409,11 +409,13 @@ struct Options {
 
   // Reading a single block from a file can cause the OS/FS to start
   // readaheads of other blocks from the file. Default: true
+  // Note: Deprecated
   bool allow_readahead;
 
   // The reads triggered by compaction allows data to be readahead
   // by the OS/FS. This overrides the setting of 'allow_readahead'
   // for compaction-reads. Default: true
+  // Note: Deprecated
   bool allow_readahead_compactions;
 
   // Allow the OS to mmap file for reading. Default: false