Rename "handle" to "cookie" in nbd client

[nirs]

Following this NBD patch: https://lists.debian.org/nbd/2023/03/msg00009.html

The term handle is confusing with handle_* functions. Renaming it will make it easier to navigate and search the code.

Lets wait until this merged upstream, since current code matches the NBD spec: https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md