Implement memory pages and write back mechanism

[tigercosmos]

Now we use dirty property to record the status of the SQL object.
The pool stores many SQL, and when the SQL is pop out or the client disconnects the server, the dirty data will write back to storage.

We should rather implement memory pages and manages the pages. Probably use memmap crate.
We should also write back data as soon as possible. The time written back needs discuss.

[tigercosmos]

https://github.com/jonhoo/rust-evmap

GitHub

jonhoo/rust-evmap

A lock-free, eventually consistent, concurrent multi-value map. - jonhoo/rust-evmap