Merge bitcoin#27626: Parallel compact block downloads, take 3

This is a backport of bitcoin#27626

This PR attempts to mitigate a single case, where high bandwidth peers
can bail us out of a flakey peer not completing blocks for us. We allow
up to 2 additional getblocktxns requests per unique block. This would
hopefully allow the chance for an honest high bandwidth peer to hand us
the transactions even if the first in flight peer stalls out.

Changes:
- Convert mapBlocksInFlight from std::map to std::multimap to allow
  multiple peers to be downloading the same block
- Add IsBlockRequestedFromOutbound() to check if block is being
  fetched from an outbound peer
- Update RemoveBlockRequest() to take optional from_peer parameter
  to selectively remove only that peers request
- Add MAX_CMPCTBLOCKS_INFLIGHT_PER_BLOCK = 3 constant
- Remove nBlocksInFlight counter, use vBlocksInFlight.size() instead
- Add parallel download test in p2p_compactblocks.py

Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

src/net.h

@@ -221,7 +221,9 @@ class CNodeStats
     int nVersion;
     std::string cleanSubVer;
     bool fInbound;
+    // We requested high bandwidth connection to peer
     bool m_bip152_highbandwidth_to;
+    // Peer requested high bandwidth connection
     bool m_bip152_highbandwidth_from;
     int m_starting_height;
     uint64_t nSendBytes;