Implemeted checksums so that sqlite3 is able to read our WAL. This also
helps with future work on proper recovery of WAL.
Create some frames with CREATE TABLE and kill the process so that there
is no checkpoint.
```
Limbo v0.0.6
Enter ".help" for usage hints.
limbo> create table x(x);
limbo> [1] 15910 killed cargo run xlimbo.db
```
Now sqlite3 is able to recover from this WAL created in limbo:
```
sqlite3 xlimbo.db
SQLite version 3.43.2 2023-10-10 13:08:14
Enter ".help" for usage hints.
sqlite> .schema
CREATE TABLE x (x);
```
Since pages are now tracked with a single centralized structure, it is
possible for a page to have been unloaded a be kept in memory for
metadata purposes like going back in the stack.
Using the example of going to a parent page which was unloaded for
whatever reason: in this case we need to check if it's loaded, if not,
we load it. Locked still means the same thing, loaded just means the
contents of a page are present in memory and if they are present, they
must be in cache.
