validation: make IsInitialBlockDownload() lock-free

`ChainstateManager::IsInitialBlockDownload()` is queried on hot paths and previously acquired `cs_main` internally, contributing to lock contention.

Cache the IBD status in `m_cached_is_ibd`, and introduce `ChainstateManager::UpdateIBDStatus()` to latch it once block loading has finished and the current chain tip has enough work and is recent.
Call the updater after tip updates and after `ImportBlocks()` completes.

Since `IsInitialBlockDownload()` no longer updates the cache, drop `mutable` from `m_cached_is_ibd` and only update it from `UpdateIBDStatus()` under `cs_main`.

Update the new unit test to showcase the new `UpdateIBDStatus()`.

Co-authored-by: Patrick Strateman <patrick.strateman@gmail.com>
Co-authored-by: Martin Zumsande <mzumsande@gmail.com>

src/init.cpp

@@ -1966,6 +1966,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         ScheduleBatchPriority();
         // Import blocks and ActivateBestChain()
         ImportBlocks(chainman, vImportFiles);
+        WITH_LOCK(::cs_main, chainman.UpdateIBDStatus());
         if (args.GetBoolArg("-stopafterblockimport", DEFAULT_STOPAFTERBLOCKIMPORT)) {
             LogInfo("Stopping after block import");
             if (!(Assert(node.shutdown_request))()) {