Support deletion of database

[MathieuBordere]

SQLite doesn't support the "DROP database" SQL statement and says to delete the database file instead. In dqlite's case it lives in memory. SQLite's xDelete VFS method to delete the in-memory file is implemented in dqlite, but can't be accessed through the go-dqlite client.

We should provide a way for a user to delete a database so that after reopening a deleted database, the database is pristine.