2.16.0
Motivation: Today, we just expect the ELGs passed to the bootstraps to be the correct ones, if not, we crash. Modifications: Offer an alternative `validatingGroup:` `init` that just returns `nil` if the ELGs are of the wrong types. Result: Easier to work with multi-stack systems for example when the user might pass an ELG for either NIO on Sockets or NIO on Network.framework.