Refactor of the virtual wal API. Drop global wals in favor of passing
the wal implementation as an argument to libsql_open.
The WAL interface is split in two sets of virtual methods:
- `create_wal` is passed when opening a sqlite connection. It's role is
to instantiate a wal.
- `libql_wal` is the wal itself, created by the `create_wal`.
The sqlite3_wal and `sqlite3_create_wal` implementation are completely
decoupled from the wal implementation. They are the default
implementation when using the traditional open methods, and
sqlite3_create_wal is exposed as a global variable, and can be composed
with other wal implementations.
