remove malloc when create data and index iterator in Get

Summary:
  Define Block::Iter to be an independent class to be used by block_based_table_reader
  When creating data and index iterator, update an existing iterator rather than new one
  Thus malloc and free could be reduced

Benchmark,
Base:
commit 76286ee67e
commands:
--db=/dev/shm/rocksdb --num_levels=6 --key_size=20 --prefix_size=20 --keys_per_prefix=0 --value_size=100 --write_buffer_size=134217728 --max_write_buffer_number=2 --target_file_size_base=33554432 --max_bytes_for_level_base=1073741824 --verify_checksum=false --max_background_compactions=4 --use_plain_table=0 --memtablerep=prefix_hash --open_files=-1 --mmap_read=1 --mmap_write=0 --bloom_bits=10 --bloom_locality=1 --memtable_bloom_bits=500000 --compression_type=lz4 --num=2621440 --use_hash_search=1 --block_size=1024 --block_restart_interval=1 --use_existing_db=1 --threads=1 --benchmarks=readrandom —disable_auto_compactions=1

malloc: 3.30% -> 1.42%
free: 3.59%->1.61%

Test Plan:
  make all check
  run db_stress
  valgrind ./db_test ./table_test

Reviewers: ljin, yhchiang, dhruba, igor, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20655

table/block_based_table_reader.h

@@ -23,6 +23,7 @@
 namespace rocksdb {
 
 class Block;
+class BlockIter;
 class BlockHandle;
 class Cache;
 class FilterBlockReader;
@@ -111,8 +112,10 @@ class BlockBasedTable : public TableReader {
   bool compaction_optimized_;
 
   class BlockEntryIteratorState;
+  // input_iter: if it is not null, update this one and return it as Iterator
   static Iterator* NewDataBlockIterator(Rep* rep, const ReadOptions& ro,
-                                        const Slice& index_value);
+                                        const Slice& index_value,
+                                        BlockIter* input_iter = nullptr);
 
   // For the following two functions:
   // if `no_io == true`, we will not try to read filter/index from sst file
@@ -120,6 +123,8 @@ class BlockBasedTable : public TableReader {
   CachableEntry<FilterBlockReader> GetFilter(bool no_io = false) const;
 
   // Get the iterator from the index reader.
+  // If input_iter is not set, return new Iterator
+  // If input_iter is set, update it and return it as Iterator
   //
   // Note: ErrorIterator with Status::Incomplete shall be returned if all the
   // following conditions are met:
@@ -127,7 +132,8 @@ class BlockBasedTable : public TableReader {
   //  2. index is not present in block cache.
   //  3. We disallowed any io to be performed, that is, read_options ==
   //     kBlockCacheTier
-  Iterator* NewIndexIterator(const ReadOptions& read_options);
+  Iterator* NewIndexIterator(const ReadOptions& read_options,
+                             BlockIter* input_iter = nullptr);
 
   // Read block cache from block caches (if set): block_cache and
   // block_cache_compressed.