Ability to configure bufferedio-reads, filesystem-readaheads and mmap-read-write per database.

Summary:
This patch allows an application to specify whether to use bufferedio,
reads-via-mmaps and writes-via-mmaps per database. Earlier, there
was a global static variable that was used to configure this functionality.

The default setting remains the same (and is backward compatible):
 1. use bufferedio
 2. do not use mmaps for reads
 3. use mmap for writes
 4. use readaheads for reads needed for compaction

I also added a parameter to db_bench to be able to explicitly specify
whether to do readaheads for compactions or not.

Test Plan: make check

Reviewers: sheki, heyongqiang, MarkCallaghan

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D9429

db/transaction_log_iterator_impl.h

@@ -10,6 +10,7 @@
 #include "leveldb/transaction_log_iterator.h"
 #include "db/log_file.h"
 #include "db/log_reader.h"
+#include "util/storage_options.h"
 
 namespace leveldb {
 
@@ -27,6 +28,7 @@ class TransactionLogIteratorImpl : public TransactionLogIterator {
  public:
   TransactionLogIteratorImpl(const std::string& dbname,
                              const Options* options,
+                             const StorageOptions& soptions,
                              SequenceNumber& seqNum,
                              std::vector<LogFile>* files,
                              SequenceNumber const * const lastFlushedSequence);
@@ -47,6 +49,7 @@ class TransactionLogIteratorImpl : public TransactionLogIterator {
  private:
   const std::string& dbname_;
   const Options* options_;
+  const StorageOptions& soptions_;
   const uint64_t sequenceNumber_;
   const std::vector<LogFile>* files_;
   bool started_;