Remove deprecated static ifs in RequestMap.addHandler

[tiyash-basu-sociomantic]

LGTM. All tests pass with dhtproto.

[matthias-wende-sociomantic]

Same idea as Tiyash had (https://github.com/sociomantic-tsunami/dlsproto/pull/110#discussion_r285024023). We might want to add (final) getter/setter functions to the IRequest interface to enforce derived classes to add the timing and scheduled_for_removal members.

[gavin-norman-sociomantic]

The reason for not being able to add these fields to the interface is that they're static. They're used at run-time in the addHandler template to check for certain properties of the request type.