[release-v1.8] vsp: Hardcode background ctx

The VSP client code was not handling contexts correctly. It was
receiving short-lived contexts bound to the lifespan of a single RPC
request, and using them to create feePayment instances. Most of the code
in feePayment executes async, so should be running on a long-lived
context such as context.Background().

Backport of 19f7ed1

internal/vsp/feepayment.go

@@ -212,7 +212,7 @@ func (c *Client) feePayment(ctx context.Context, ticketHash *chainhash.Hash, pai
 
 	fp = &feePayment{
 		client:     c,
-		ctx:        ctx,
+		ctx:        context.Background(),
 		ticketHash: *ticketHash,
 		policy:     c.policy,
 	}