Fix WeightsSkipList: faithful port of p2pool lazy level-building algorithm

The skip list was producing wrong PPLNS weights causing persistent
GENTX-MISMATCH on mainnet. Three bugs fixed:

1. Eager level building replaced with p2pool's lazy "updates" mechanism
   (skiplist.py:24-34,54-56) — higher levels are built during query
   walks, not at node construction time, ensuring correct combined deltas.

2. Missing updates propagation after jumps — after taking a skip-list
   jump, the accumulated delta must be combined into all pending update
   entries so future level fill-ins get correct aggregate weights.

3. std::vector replaced with std::deque for skip levels — push_back
   during lazy level building was invalidating references to existing
   level deltas held by the query walk.

Verified: 0 GENTX-MISMATCH on testnet node 33 (535+ shares verified),
all 198 built tests pass, PPLNS query time 3.3ms at depth 550.

Author: frstrtr