[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/transaction_log_iterator_impl.h

@@ -10,7 +10,6 @@
 #include "leveldb/transaction_log_iterator.h"
 #include "db/log_file.h"
 #include "db/log_reader.h"
-#include "util/storage_options.h"
 
 namespace leveldb {
 
@@ -26,7 +25,7 @@ class TransactionLogIteratorImpl : public TransactionLogIterator {
  public:
   TransactionLogIteratorImpl(const std::string& dbname,
                              const Options* options,
-                             const StorageOptions& soptions,
+                             const EnvOptions& soptions,
                              SequenceNumber& seqNum,
                              std::unique_ptr<std::vector<LogFile>> files,
                              SequenceNumber const * const lastFlushedSequence);
@@ -42,7 +41,7 @@ class TransactionLogIteratorImpl : public TransactionLogIterator {
  private:
   const std::string& dbname_;
   const Options* options_;
-  const StorageOptions& soptions_;
+  const EnvOptions& soptions_;
   const uint64_t startingSequenceNumber_;
   std::unique_ptr<std::vector<LogFile>> files_;
   bool started_;