accept recv-only channels for all cancellations

[jbardin]

Cancellation channels are often derived from a Context, which
returns a directional <-chan struct{} from Done(). In order to use
this with parts of the consul API, one is required to create a new channel
and dispatch a separate goroutine to watch for context cancellation and
close the new channel.

Changing the signature for the methods that take cancellation channels
will allow easier integration with existing uses of Context. Since the
cancellation pattern only reads from these channels, there should be no
backwards incompatibility with existing codebases, and most of the
methods already accept only the correct type.

[slackpad]

LGTM - merged and added to the change log.