DhcpV4Client trait for async and blocking client

[Jackbaude]

The DhcpV4ClientAsync does not implement the same methods as DhcpV4Client. It would be helpful to have methods such as release_lease on the async version. Is there a way to call these methods on DhcpV4Client or would this be helpful to additionally have these methods on DhcpV4ClientAsync. I would be willing to do this if that is the case.

[cathay4t]

The underneath implementation is just send out a UDP package and no reply required from DHCP server.

So you may just create a async function to DhcpV4ClientAsync without any async stuff in it just to be consistent with DhcpV4Client.

I have to admit, this project is crated before rust async become stable, so the fundamental code is not async-aware.