Include all channel route hints if no connected channels exist

Include all channel route hints if no connected channels exist

Mobile clients often take a second or two before they reconnect to
their peers as its not always clear immediately that connections
have been killed (especially on iOS). This can cause us to
spuriously fail to include route hints in our invoices if we're on
mobile.

The fix is simple, if we're selecting channels to include in route
hints and we're not not connected to the peer for any of our
channels, we should simply include the hints for all channels, even
though we're disconencted.

Fixes #1768.