Secure delete by default

[brodycj]

According to https://www.sqlite.org/pragma.html#pragma_secure_delete:

The default setting for secure_delete is determined by the SQLITE_SECURE_DELETE compile-time option and is normally off. The off setting for secure_delete improves performance by reducing the amount of disk I/O.

while according to https://www.w3.org/TR/webdatabase/#sensitivity-of-data:

[...] user agents should ensure that when deleting data, it is promptly deleted from the underlying storage.