Select free ports for lnd from a pool of options. By default we will
use the standard lnd ports (10009 and 9735) but if those are not
available then we will search for free ports in a nearby range.
This allows us to have multiple instances of the wallet running and
avoids possible conflicts with other instances of lnd that the user
may be running.
Lnd doesn't currently do a graceful shutdown when a `SIGTERM` is issued.
To ensure a graceful shutdown, call `stopDaemon` on the gRPC interface
prior to terminating lnd. If a graceful shutdown is unsuccessful then
force kill the daemon.
Tom Kirkpatrick