Change MinSwapAmountMsat to be set by policy.

Added test case to confirm that swap fails
on StuckChannels.
Allow pushAmt to be set during setup.
Set min_swap_amount_msat to 1.

Replace sanity check with probe payment in SpendableMsat.
ProbePayment trying to pay via a route with
 a random payment hash that the receiver doesn't have the preimage of.
The receiver node aren't able to settle the payment.
When the probe is successful, the receiver will return
a incorrect_or_unknown_payment_details error to the sender.
If the result of ProbePayment is assumed
to be a payment failure, log the reason as known by ProbePayment

Route may not be built even when channel is active.
Since this causes flaky test,
Added to check route construction in advance.

Restored sanity check of the maximum htlc limit.
* To check max htlc more strictly than CLN's own limits.
* To prevents a tiny bit of database bloat
in situations where it is guaranteed to fail.

To ensures that the invoice payment is probed
via the direct channel to the peer.