Update validate function to return default target for zero value
confirmation targets, as indicated by the comment. This change
introduces a behaviour change for direct rpc calls to loopd (ie,
those not made by the loop cli tool). Previously, these calls
would fail and indicate that the conf target must be > 2, now
they will succeed with the default conf target. The loop cli
tool is unaffected because we already set the default value.
When clients upgrade from a previous version of loopd which did
not have this check, preexisting loops will be unaffected, because
loop ins had the default of 6 confirmations set, and loop outs with
<2 conf target would not have been created.
This allows Loop users to further improve their privacy by not revealing
their source IP address. Note that the identity of the lnd node behind
Loop can still be revealed when performing a Loop In due to the swap
server extending an off-chain HTLC to the user.
Onion addresses don't yet exist for the swap servers, but they will be
added at a later time.
A database field was already in place to allow channel selection for
loop in. Unfortunately this field, which contains a short channel id,
isn't easily usable for controlling the loop server payment. Because of
non-strict forwarding, it is only possible to constrain a route to a
specific last hop pubkey.
This commit converts the existing field into a pubkey field.
