This commit adds verification to the loop out request to ensure that the
formatting of the specified destination address matches the network that
lnd is running on.
This commit moves loop out request validation for labels and
confirmation targets into its own function for the purpose of easy
testing and also to make the additions of future request validation easy
to add and test.
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.
