Implement DHCP lease maintenance with renew / rebind.

[xorbit]

After initializing everything, the DHCP lease can now be maintained from the main loop by calling eth.maintain_dhcp_lease(). An example main loop for a WSGI web server + DHCP maintenance can look like this:

while True:
    # Our main loop where we have the server poll for incoming requests
    wsgiServer.update_poll()
    # Maintain DHCP lease
    eth.maintain_dhcp_lease()

• Now can use socket.bind to set source port to be consistent with CPython socket behavior.
• DHCP object is now persistent in WIZNET5K because we want to keep it around to automatically renew and rebind.
• Added maintain_dhcp_lease to WIZNET5K to properly maintain the DHCP lease with renew / rebind behavior.
• Implement correct DHCP transaction ID behavior so we don't accidentally grab an IP that was supposed to be assigned to another device.
• Now keeps track of automatically assigned source ports to prevent duplication.
• Renamed WIZNET5K._src_port to WIZNET5K.src_port because it's externally accessed from socket.connect.
• Removed setting source port for DNS lookup, moved to bind before socket connect.
• WIZNET5K.get_socket now only returns fully closed sockets. Previous behavior caused problems with stray old data from previous connections confusing new connections when the socket wasn't fully closed yet.
• Improved WSGI server in how it deals with socket removal and allocation.
• Fixed some bugs and comments.

[brentru]

@xorbit Great work and thank you for the PR, I'll review and test on hardware this week.

[brentru]

@xorbit Thank you for the PR, I tested the WSGI and BTC handler locally with an Ethernet FeatherWing and it works great!