routerrpc: implement LSP heuristic and multi-LSP worst-case probing

This commit implements a comprehensive LSP (Lightning Service Provider)
detection heuristic and updates the payment probing logic to handle
multiple LSPs with worst-case fee estimation.

Key changes:

1. LSP Detection Heuristic (isLSP function):
   Implements three rules to detect LSP setups:
   - Rule 1: If invoice target is public → NOT an LSP (route directly)
   - Rule 2: If at least one destination hop is public → IS an LSP
   - Rule 3: If all destination hops are private → NOT an LSP

2. LSP Route Preparation (prepareLspRouteHints function):
   - Groups route hints by unique public LSP nodes
   - Filters out non-LSP routes based on the heuristic
   - Tracks worst-case fees and CLTV delays for each LSP
   - Returns adjusted route hints with LSP hop stripped

3. Multi-LSP Probing (probePaymentRequest updates):
   - Probes up to 3 unique LSPs maximum (griefing protection)
   - Selects the WORST-CASE (most expensive) route for conservative
     fee estimation
   - Adds comprehensive debug logging for worst-case selection process
   - Properly formats vertex logging using %v (calls Vertex.String())

The worst-case approach ensures users won't be surprised by higher fees
when the actual payment is sent, providing a more conservative and
reliable fee estimate.

Author: ziggie1984