Handle exceptions during receive and dispose timer

[Nuklon]

Exceptions weren't handled properly, causing them to appear in UnobservedTaskException.
Also the Timer created during MainTask wasn't disposed. I've split this up in two methods to greatly simplify the logic.

[jpmikkers]

Hi, I agree the OperationCanceledException should be handled of course (I've been working on asyncifying TFTPServer and ran into the same issue). But there's a good reason to have just one thread/maintask: this ensures I don't need locking for state updates of the dhcp server. I'll have a go at merging the fixes I applied to TFTPServer into DHCPServer, that should do the trick.

[Nuklon]

Ah, yes, I see. The problem is that with WhenAny one task remains running in the background and never catched properly. I think it makes sense to add some trivial synchronization here, I've added another commit.