[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

db/table_cache.h

@@ -14,7 +14,6 @@
 #include "leveldb/cache.h"
 #include "port/port.h"
 #include "table/table.h"
-#include "util/storage_options.h"
 
 namespace leveldb {
 
@@ -23,7 +22,7 @@ class Env;
 class TableCache {
  public:
   TableCache(const std::string& dbname, const Options* options,
-             const StorageOptions& storage_options, int entries);
+             const EnvOptions& storage_options, int entries);
   ~TableCache();
 
   // Return an iterator for the specified file number (the corresponding
@@ -58,7 +57,7 @@ class TableCache {
   Env* const env_;
   const std::string dbname_;
   const Options* options_;
-  const StorageOptions& storage_options_;
+  const EnvOptions& storage_options_;
   std::shared_ptr<Cache> cache_;
 
   Status FindTable(const EnvOptions& toptions,