SST相关的修改

[yangyang233333]

1. 完成SSTable模块;
2. 修改src/db/offset_info.h，增加clear()函数; 3.
   修改src/table/data_block_builder.*文件，增加clear()、size()函数，以及其他修改;
3. 修改filter_block_builder.*文件;
4. 增加FooterBuilder类;
5. 增加IndexBlockBuilder类;
6. 相关单元测试;
7. 其他必要修改;

改动的文件如下:
modified: CMakeLists.txt
modified: README.md
modified: build.sh
new file: code_statistics.sh
modified: docker/Dockerfile
new file: img/logo.png
modified: src/db/offset_info.h
modified: src/filter/bloom_filter.cpp
modified: src/filter/bloom_filter.h
modified: src/filter/filter_policy.h
modified: src/table/data_block_builder.cpp
modified: src/table/data_block_builder.h
new file: src/table/filter_block_builder.cpp
new file: src/table/filter_block_builder.h
new file: src/table/footer_builder.cpp
new file: src/table/footer_builder.h
new file: src/table/index_block_builder.cpp
new file: src/table/index_block_builder.h
deleted: src/table/restart_point.h
new file: src/table/sstable_builder.cpp
new file: src/table/sstable_builder.h
modified: tests/test_data_block_builder.cpp
new file: tests/test_filter_block_builder.cpp
new file: tests/test_index_block_builder.cpp
modified: unittest_run.sh