Make arena block size configurable

Summary:
Add an option for arena block size, default value 4096 bytes. Arena will allocate blocks with such size.

I am not sure about passing parameter to skiplist in the new virtualized framework, though I talked to Jim a bit. So add Jim as reviewer.

Test Plan:
new unit test, I am running db_test.

For passing paramter from configured option to Arena, I tried tests like:

  TEST(DBTest, Arena_Option) {
  std::string dbname = test::TmpDir() + "/db_arena_option_test";
  DestroyDB(dbname, Options());

  DB* db = nullptr;
  Options opts;
  opts.create_if_missing = true;
  opts.arena_block_size = 1000000; // tested 99, 999999
  Status s = DB::Open(opts, dbname, &db);
  db->Put(WriteOptions(), "a", "123");
  }

and printed some debug info. The results look good. Any suggestion for such a unit-test?

Reviewers: haobo, dhruba, emayanke, jpaton

Reviewed By: dhruba

CC: leveldb, zshao

Differential Revision: https://reviews.facebook.net/D11799

db/memtable.h

@@ -11,8 +11,8 @@
 #include "db/dbformat.h"
 #include "db/skiplist.h"
 #include "db/version_set.h"
-#include "util/arena.h"
 #include "leveldb/memtablerep.h"
+#include "util/arena_impl.h"
 
 namespace leveldb {
 
@@ -29,8 +29,11 @@ class MemTable {
 
   // MemTables are reference counted.  The initial reference count
   // is zero and the caller must call Ref() at least once.
-  explicit MemTable(const InternalKeyComparator& comparator,
-    std::shared_ptr<MemTableRepFactory> table_factory, int numlevel = 7);
+  explicit MemTable(
+    const InternalKeyComparator& comparator,
+    std::shared_ptr<MemTableRepFactory> table_factory,
+    int numlevel = 7,
+    const Options& options = Options());
 
   // Increase reference count.
   void Ref() { ++refs_; }
@@ -101,7 +104,7 @@ class MemTable {
 
   KeyComparator comparator_;
   int refs_;
-  Arena arena_;
+  ArenaImpl arena_impl_;
   shared_ptr<MemTableRep> table_;
 
   // These are used to manage memtable flushes to storage