Make the block-based table's index pluggable

Summary:
This patch introduced a new table options that allows us to make
block-based table's index pluggable.

To support that new features:

* Code has been refacotred to be more flexible and supports this option well.
* More documentation is added for the existing obsecure functionalities.
* Big surgeon on DataBlockReader(), where the logic was really convoluted.
* Other small code cleanups.

The pluggablility will mostly affect development of internal modules
and won't change frequently, as a result I intentionally avoid
heavy-weight patterns (like factory) and try to make it simple.

Test Plan: make all check

Reviewers: haobo, sdong

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16395

table/block_based_table_factory.cc

@@ -11,7 +11,10 @@
 #include "table/block_based_table_factory.h"
 
 #include <memory>
+#include <string>
 #include <stdint.h>
+
+#include "rocksdb/flush_block_policy.h"
 #include "table/block_based_table_builder.h"
 #include "table/block_based_table_reader.h"
 #include "port/port.h"
@@ -40,12 +43,8 @@ Status BlockBasedTableFactory::NewTableReader(
 TableBuilder* BlockBasedTableFactory::NewTableBuilder(
     const Options& options, const InternalKeyComparator& internal_comparator,
     WritableFile* file, CompressionType compression_type) const {
-  auto flush_block_policy_factory =
-      table_options_.flush_block_policy_factory.get();
-
-  auto table_builder =
-      new BlockBasedTableBuilder(options, internal_comparator, file,
-                                 flush_block_policy_factory, compression_type);
+  auto table_builder = new BlockBasedTableBuilder(
+      options, table_options_, internal_comparator, file, compression_type);
 
   return table_builder;
 }
@@ -55,4 +54,7 @@ TableFactory* NewBlockBasedTableFactory(
   return new BlockBasedTableFactory(table_options);
 }
 
+const std::string BlockBasedTablePropertyNames::kIndexType =
+    "rocksdb.block.based.table.index.type";
+
 }  // namespace rocksdb