- Add user-space callback for the authorizer function (this is potentially
  very slow, so it needs to be implemented carefully).

- Add user-space callback to handle busy databases.

- Test-suite

- If building a ZTS build, -DTHREADSAFE while compiling libsqlite

- If building a non-debug build, -DNDEBUG will disable the expensive
  asserts in the inner loop. (and double performance)

vim:tw=78