Avoid callbacks when opening connection pools

[simolus3]

NativeCallable and Pointer.fromFunction are implemented by the Dart VM allocating an FFI stub in a page that is then remapped as executable. This is forbidden on some platforms like GrapheneOS.

To avoid that issue, this splits the pkg_sqlite3_connection_pool_open function into three parts:

1. pkg_sqlite3_connection_pool_open locks the global mutex of active pools and either returns a handle to the opened pool if it exists already, or indicates that the caller is responsible for initialization.
2. pkg_sqlite3_connection_pool_initialize takes the role of what used to be a callback, turning an initialized pool into a pool client and unlocking.
3. pkg_sqlite3_connection_pool_close_uninitialized drops an uninitialized handle without creating a pool, used on exceptions.

Closes #396.

[simolus3]

Merged in f1cd435.