Remove unused memory backing store, page cache, pager, and related tests

- Deleted MemoryBackingStore implementation and its associated tests.
- Removed PageCache and Pager implementations along with their tests.
- Eliminated SlotIndexEntry structure and its related code.
- Cleaned up unused SystemInfo struct and its functionality.
- Removed WAL integration tests and stored procedures tests as they are not fully implemented.

Author: JEleniel

.github/CODEOWNERS

@@ -54,3 +54,6 @@ EpilogLite is a pure Rust database library implementation inspired by SQLite, de
 - Do not invent facts or misrepresent repository state; ask maintainers when uncertain.
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