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

table/table_test.cc

@@ -257,7 +257,7 @@ class TableConstructor: public Constructor {
     // Open the table
     uniq_id_ = cur_uniq_id_++;
     source_.reset(new StringSource(sink_->contents(), uniq_id_));
-    return Table::Open(options, std::move(source_),
+    return Table::Open(options, soptions, std::move(source_),
                        sink_->contents().size(), &table_);
   }
 
@@ -271,7 +271,7 @@ class TableConstructor: public Constructor {
 
   virtual Status Reopen(const Options& options) {
     source_.reset(new StringSource(sink_->contents(), uniq_id_));
-    return Table::Open(options, std::move(source_),
+    return Table::Open(options, soptions, std::move(source_),
                        sink_->contents().size(), &table_);
   }
 
@@ -295,6 +295,7 @@ class TableConstructor: public Constructor {
   TableConstructor();
 
   static uint64_t cur_uniq_id_;
+  const StorageOptions soptions;
 };
 uint64_t TableConstructor::cur_uniq_id_ = 1;