lnd: turn off channel validation for Neutrino by default
lnd: turn off channel validation for Neutrino by default

Downloading every block that contains a channel point takes a very long
time when syncing the graph on mainnet with Neutrino. Therefore it makes
sense to use routing.assumechanvalid=true since by using Neutrino a user
already accepts the different trust model.
Apparently the existence or meaning of the routing.assumechanvalid flag
is unknown to a lot of users and is overlooked.
This commit basically sets the default to routing.assumechanvalid=true
for Neutrino. Because the CLI library doesn't support setting a bool
value to false by the user if the default is true, we need to add an
additional flag that is the inverse of the routing one, just for the
case where a Neutrino user explicitly wants to turn on channel
validation.

aakselrod/lnd,aakselrod/lnd