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

include/rocksdb/table.h

@@ -54,6 +54,21 @@ struct BlockBasedTableOptions {
   // If not specified, each "table reader" object will pre-load index/filter
   // block during table initialization.
   bool cache_index_and_filter_blocks = false;
+
+  // The index type that will be used for this table.
+  enum IndexType : char {
+    // A space efficient index block that is optimized for
+    // binary-search-based index.
+    kBinarySearch,
+  };
+
+  IndexType index_type = kBinarySearch;
+};
+
+// Table Properties that are specific to block-based table properties.
+struct BlockBasedTablePropertyNames {
+  // value of this propertis is a fixed int32 number.
+  static const std::string kIndexType;
 };
 
 // Create default block based table factory.