Fix delays in pcap implementation

[jethrogb]

The previous implementation had several issues:

• pcap-async relied on a blocking function (smol::unblock) internally making it not really async
  • I also noticed packet processing delays related to the buffer_for Duration, not sure if this was related or not.
• Blocking MPSC channels were used in async code

These are fixed by switching to the pcap crate which has since gotten its own async implementation. I also added an async implementation for the other direction.

Depends on rust-pcap/pcap#372 and rust-pcap/pcap#373