refactor: separate `{Wal,Main,DiskMain}File`s behaviour

[marco6]

This PR further separates the behaviour of the many different files SQLite uses. In detail, it splits the behaviour in 3 different structs:

• vfsWalFile which is only related to WAL operation. This file is always in memory for dqlite
• vfsMainFile the real database file. This file is always in memory for dqlite. This file also indirectly manages the shared memory (-shm file).
• vfsDiskMainFile again, the real database file, but in disk mode. This file is on disk.