Add -maxblocksintransit: tunable per-peer block-download window

The per-peer in-flight block window (MAX_BLOCKS_IN_TRANSIT_PER_PEER) was a
hardcoded 16. On a single, high-latency peer during IBD the transfer is
bandwidth-delay-product bound (window / RTT), so with tiny sub-checkpoint
blocks the window, not bandwidth, is the ceiling — measured ~4x throughput
going 16 -> 64 on a 350ms-RTT peer. Make it a runtime flag (default 16,
clamped 1..4096), logged at startup. No behavior change at the default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/main.cpp

@@ -90,6 +90,7 @@ static int64_t nTimeBestReceived = 0;
 CWaitableCriticalSection csBestBlock;
 CConditionVariable cvBlockChange;
 int nScriptCheckThreads = 0;
+int MAX_BLOCKS_IN_TRANSIT_PER_PEER = DEFAULT_MAX_BLOCKS_IN_TRANSIT_PER_PEER;
 int nRandomXVerifyThreads = 0; // parallel RandomX pre-verification worker count (0 = inline only)
 bool fExperimentalMode = true;
 bool fImporting = false;