Add support for batching NfNetlink messages

[Toorero]

This MR should resolve #43.

It introduces a new ConnectionHandle::request_batch method that supports sending multiple messages per request (and datagram by extension).

Example Usage

mod_set_entry_proto.rs

[cathay4t]

The changes to netlink-proto are forcing other rust-netlink crates to bump their API version and other external users might need to change their working code.

Can we somehow create new API instead changing API?

[Toorero]

Can you further elaborate on your concern?

1. The changes are backwards compatible. So external crates don't need to change their working code. The public facing API is only extended by ConnectionHandle::request_batch.
2. Why would other rust-netlink crates need to change their API version? Do they export ConnectionHandle::request_batch?

[cathay4t]

Can you further elaborate on your concern?

1. The changes are backwards compatible. So external crates don't need to change their working code. The public facing API is only extended by `ConnectionHandle::request_batch`.

2. Why would other rust-netlink crates need to change their API version? Do they export `ConnectionHandle::request_batch`?

My bad. The mut changes is not API breaking. Approved now. Thanks for the contribution!

[Toorero]

Pipeline (rustfmt, minimal supported Rust version) fixed.