support more memory ext

[sqxccdy]

您好, 我的英语不是很好, 我用谷歌翻译并且将意思尽量简短.

�我完整的看完了有关你写代码, 准备嵌入到我有关数据存储方面里面去，但是发现如果存在多个子树并不是很友好, 所以我对有关memory部分做了如下扩展

1. 抽象有关文件操作的部分 BaseMemory
2. 删除有关wal锁的部分，跟回滚有关的代码可能 需要完善
3. 对文件操作方式未做更改, 您更多的为了文件分页做了足量的优化
4. �将配置全局化，这样子可以有效的生成多个b+树的时候配置的统一

b+树仅仅只是数据存储中很小的一部分，为了真正的使用它，而不是作为学习的玩具. 这部分扩展希望您可以参阅我的代码，更新这部分代码，或者merge...

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Hello, my English is not very good, I use Google Translate and keep the meaning as short as possible.

I completely read about the code you wrote, ready to embed into my data storage, but found that if there are multiple subtrees, it is not very friendly, so I made the following extensions to the memory part.

1. Abstract part of the file operation BaseMemory.
2. Remove the part about the wal lock, the code related to the rollback may be ** need to be improved**.
3. There are no changes to the way the file is operated. You have done more optimization for file paging.
4. Globalize the configuration so that the configuration can be efficiently generated when multiple b+ trees are generated.

The b+ tree is just a small part of the data store, in order to actually use it, not as a learning toy. This part of the extension hopes you can refer to my code, update this part of the code, or merge it...

[sqxccdy]

@NicolasLM unit test file is tests/test_extmemory.py..